It sounds like you've covered all your bases; the code seems to work since the button shows up, you've gone through Firefox settings, and disabled add-ons.

The only thing that comes to mind now is trying a new profile or Refreshing Firefox to reset things. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

I don't know what the current Mac OS version is, but could you check if you're up to date? 40.0 sounds low.

Sorry, should have said I'm on Mac 10.9.5 and Firefox 40.0.3 (set to update automatically, but when I just checked it there was a little box saying "Restart Firefox to Update". Never saw that before). I've also discovered that older videos I posted that DID go to fullscreen no longer do, giving me the same error message. I'm going to restart and see what happens.

OK, very strange. I restated Firefox and now I'm on 41.0.1. So not only did it not automatically update (I double checked and it's still on the auto update setting) but I started it up fresh this morning after shutting everything down last night and it obviously did not update then either. Bad news is my embedded videos (both old and new) still will not go to fullscreen. Good news, checked with someone on a PC with IE and all working fine there. So it's definitely a problem with my settings, so should be fixable. I really don't want to reset Firefox (been there done that and don't like having to redo all my personal settings, home pages etc.) and I question whether that will help given an update did not help. PLUS I have the same problem with Safari, so it's not isolated to Firefox. So I'm still looking for an alternative. What settings would affect BOTH Firefox and Safari but NOT Explorer. Could it be an issue with my web hosting service (GoDaddy)?

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

Firefox has a setting for full screen mode, so maybe check that.

You can inspect and manage permissions for the domain in the currently selected tab via these steps:

• click the "Site Identity Button" (globe/padlock) on the location/address bar
• click "More Information" to open "Tools > Page Info" with the Security tab selected

Go to the Permissions tab (Tools > Page Info > Permissions) to check the permissions for the domain in the currently selected tab.

You can inspect and manage permissions for all domains on the about:permissions page.

• https://support.mozilla.org/kb/how-do-i-manage-website-permissions

These are not encrypted sites with nothing sensitive being put out there, so the Site Identity Button is only a greyed-out globe. I appreciate the response, Cor-El (very catchy, I like that - you a graphic designer from Krypton?), but you overlooked details already stated. To summarize:

1. I'm on an iMac with 10.9.5 Firefox 41.0.1 2. Fullscreen mode on my Firefox settings is enabled to "Always Allow" and all Add-Ons are set to "Always Ask" 3. The embed code does in fact contain the fullscreen function, and the fullscreen icon does appear in the bottom right of my embedded videos but just gives me an error message. 4. The problem only exists with YouTube videos I've embedded on websites; fullscreen function works fine on YouTube.com and elsewhere. 5. The problem occurs in Firefox AND Safari on my iMac. 6. Checking on a friend's PC with IE (and just now on my iPhone) fullscreen works normally.

All of that, especially the last item, tells me a setting on my iMac has changed but I just don't know what or how.

If it's something affecting two browsers, antivirus comes to mind. What security suite do you have?

Once you mentioned Safari, I did a new round of googling and landed on https://productforums.google.com/forum/#!topic/youtube/vuBQUFiev3E.

For posteriority: -- You need to add 1 extra parameter ( fs=1 ) for the button visibility, and webkitallowfullscreen to enable it on Chrome and Safari and also add mozallowfullscreen for Firefox.

Example - change this: <iframe width="420" height="315" src="http://www.youtube.com/embed/S4pbbFQl62M" frameborder="0" allowfullscreen></iframe>

to this: <iframe width="420" height="315" src="http://www.youtube.com/embed/S4pbbFQl62M?fs=1" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> --

This sounds really bizarre and it's from 2012, but it's a simple fix to try.

Thanks for the input. I'm using ClamXav (paid version) with autoupdate enabled which just occurred on 9/24 (updated to v2.8.5). Honestly don't know if this was problem before that or after. I only noticed the problem 9/30. So now can you tell me ....

1. Would ClamXav only affect embedded YouTube files on sites I've built (remember I can go fullscreen elsewhere) and 2. Your fix to change the encoding above seems rash since everything worked fine until very recently and the thread you found with that is 3 years old; but stranger things have happened. We have a running joke in the family that Murphy's Law was named after the wrong Irish family. Be it the car, and appliance or tech issues I always tell the repairman to think of the most unlikely scenario and that will be our problem (I've literally had multiple techs looking at a problem say "I have no idea what's happening; never saw that before."

Since this all seems oddly specific, I'm going after the stranger angles I've encountered. A recent AV update is suspect, if it changed the suite's setup or engine and not just the ID databases.

1. AV is responsible for messing up a lot of things in browsers if (secure) connection scanning and browser protection components are used. Usually the effect is more widespread, i.e. not isolated to embedded videos like this, but since this is on two different browsers, it is what comes to mind first. ClamXav looks very rudimentary based on the documentation on their site; I can't see any web threat protection, only local scanning (the email scanning is probably for local copies only).

This makes ClamXav an unlikely culprit.

2. I think the solution is bizarre as well. This does not seem like something that needs to be specified by parameters in 2015; however, maybe your browsers need the instructions for who-knows-why. If your installations are old, they may have become corrupt and decrepit - and then we're back to the original suggestion of Refresh/Reinstall, which is a lot more work.

Since you know the timetable, you could check what other software updates may have happened (system mainly, drivers maybe) from the appropriate logs.

PS. Have you cleared your cache? If trying different machines works, and you've actively used both Safari and Firefox on your machine, local cache may interfere. Both data and cookies need to go (even if that means relogging onto sites again).

First of all thanks to all, just went back and marked all answers 'Helpful'; no solution but you're helping me eliminate possibilities. Phox, I don't use Safari because of past compatibility issues with GoDaddy, only checked there due to this problem. Nevertheless I cleared cache and cookies on both Safari and Firefox with no results. As to ClamXav, it is pretty basic, but I just realized one change since I posted the old videos that used to work and the newest ones is that I used to have the free version of ClamXav, now I am paying for the full version. I'll try disabling that and report back. As to AV, I use and have used iWeb 2.0.4, which Apple no longer supports or updates. Looking at my logs I see that I have had updates to Flash, Shockwave and Silverlight over the past few months, but since I only just noticed a problem with the new videos, and it's been a while since I posted the older ones, it's hard to pinpoint when the problem began.

  Think restoring from a previous date with Time Machine be worth trying?

I meant AV as in antivirus. If he content creation tools are no longer updated, they shouldn't change on their own.

Time Machine would be one way to try and pinpoint when your browser(s) still worked. It does affect your whole system though, so it is a large-calibre solution to a small-scale problem. If the change was an automated one, chances are it will just happen again.

If you want to experiment, I would try making a new Firefox profile to see if defaults work. This will preserve your old profile and the rest of the system. With Refresh you can experiment further, and it will even do some of the migration for you (preserving your old profile folder still), in case you prefer the fresh profile.

A manual copy of your current profile folder won't go amiss.

One more solution down ... Reset Firefox to defaults with no change, both older and new videos will not go full screen. Since you asked, here are the two web pages with video links, the first an older video that I know worked fine, the second the newest where I first noticed the problem:

http://www.scihouston.org/Fundraisers.html

http://www.scihouston.org/Home.html

I can confirm the issue at least. Mucking about now to see if I can remedy it.

With a local embed the iFrame works just fine in Firefox, Opera, and Internet Explorer 11. Firefox and Opera reject the site implementation.

Use the code, Luke. Nothing we can do on Firefox's (or Opera's) end I think.

I tried the embed online just in case it was allowed locally... same deal, the plain iframe embed works just fine. It is definitely the code not working with new FF/Opera and probably anything that uses the same engine(s).

Whoa, you lost me .... It does or doesn't work for you? What is wrong with the code?

1. I am using the embedding code provided by YouTube and if you remember it includes "allowfullscreen></iframe>" in the coding so should work, right? 2. The older embedded video DID work on my iMac with Firefox until recently. 3. BOTH work on other Windows OS I've checked (my wife's HP laptop, a client checking their web page), BUT NOT on my daughter's Dell Inspirion laptop using IE.

To clarify:

I created an HTML file with nothing but the iframe embed code in it. Works like a charm locally (file:) and through an online server (http:) on IE 11, Firefox 41.0.1, and Opera 32.0.1948.69. So the iframe embed code is a-ok.

What this means is that something in the SCI Houston site infrastructure is probably incompatible with newer versions of Gecko/Webkit(Blink) based browsers and mucks up the fullscreen function. I could not tell at a glance what the issue could be, so it'll be up to the webmaster/site coder and content creation tools to figure this one out.

IE supports many legacy features (11 is a legacy browser at this point), but I don't know what the missing or conflicting feature could be here.

Maybe investigating all the ways fullscreen can be disabled in code would yield a clue?

PS. You could compare your iPhone and iMac: iPhones use older versions of Safari (a Webkit-based browser), so this might explain why the phone can fullscreen but your computer won't.

Using iWeb iWeb '08 v2.0.4, no recent updates (don't think Apple supports anymore) and fullscreen works on my iPhone. Only thing in common with this problem is it occurs in two sites I built with iWeb, BUT since fullscreen worked until recently in both (one is as old as iWeb itself) and works on other phones and computers I figured the problem had to be with some setting on my iMac.

I use Firefox, and won't go fullscreen in that or Safari (both latest versions)

I did Google ways to disable fullscreen in the code when this first happened (that's how I found out what enables it) and found nothing in the code I got from YouTube to cause that (and remember, it's multiple codes for multiple videos, some of which worked before, so don't see how it could be in the code itself).

Think we're at a dead end here, but thanks very much for the input.

You're welcome - just sorry I couldn't solve the thing!

It looks to me like the latest versions of Gecko and Webkit based browsers like Firefox and Safari just don't like something on that site. Same with the latest Opera. Might be due to old tools making old code that interferes with the new stuff (but not oldies like IE 11 (pre-Windows 8/10) or iPhone Safari). I tried searching for fullscreen stuff, and I got some hits in the CSS files I think, but could not make anything of it. There is some sort of box or overlay on the embed item though, so could start there - the embed snippet itself is fine, something in the surrounding code is interfering.

If you want to consider replacing iWeb, http://www.everwebapp.com/ comes recommended by MacLife and others. They even have a guide for switching over from iWeb.

Good luck!

Hmmmm, thanks again, never worried about the overlap as it never hurt anything before, but I'll take a look at the and let you know moving things makes a difference .... nope.