Does anyone have an answer to why youtube videos will not play when newly posted, but will play after they have "aged" for a while, sometimes it's as hour, sometimes it 72 hours, usually no more than this, It's almost every new video that does this, but they will always play if I wait long enough, WHY! is this??? driving me nuts, can't figure it out, please help!?

I have no issue with FF57.0.2 with youtube. What is the error your getting?

when I click on a NEW video on youtube I get the error message "your browser does not recognize any video formats available" I get this error message with all new posted video's, after a while they will all play but sometimes it hours, sometimes days before they play, why doesn't firefox recognize the video formats available while the video is a new video, but will recognized the video formats available later and play the video fine? all older videos play fine, I can upload my own videos and they will play fine, this is driving me nuts, cause it's only happening for new vids and only until the video has aged for a while.

The most common reason for this is that your Firefox can't play the format/codec used by the original poster, but after YouTube builds all the alternate formats, it is able to find one compatible with your Firefox.

As you probably know, Firefox on Windows XP cannot use the Media Foundation to decode H.264/MP4 video. (Media Foundation was only released for Vista and later.) So it would be expected that there are videos that can't be played in the HTML5 player if they are only available in that format.

Firefox is a supported browser on youtube and it should be/is compatible with most all formats used? Wouldn't these popular sites that post lots of new videos, be posting with enough formats from the start that viewers are not going to be having viewing issues/problems, and isn't that's what HTML5 is for if they don't play in the other format, how can I tell what format a video/browser is using or if it's playing in and HTML5 works? I didn't have this problem about 6 months ago, my computer works fine everywhere else, plays most all vids fine unless linked to youtube and than sometimes will not play, I do get a "video codex" error message sometimes at other sites, but it seems to be only/mainly a youtube problem, and random sometimes, I can't figure out what changed? The regularly posted videos on popular sites like Jay Leno's Garage(48 hours before I could view his latest), Motor Trend channel, TheHoonigans, and others that regularly post videos used to play fine for me, now it's most won't play until some time has passed, while a new posting on TheHoonigans that was posted about 10 hours ago, just played fine, at the same time one other video that THeHoonigans posted 48 hours ago, still will not play for me. I've watched and played other videos on youtube by other posters that were posted within the hour and are not "aged" at all, they play just fine, but I'll try another vid from another poster and it won't play, this crazy randomness and time delay before vids play is the part of my problem that doesn't make any sense knowing about formats and such, that's the part I can't figure out!?

Let me start with this: H.264/MP4 is a patented technology. For one reason or another, Mozilla has not licensed the patent. It might be incompatible with Firefox being an open source application, I don't know.

On every other major platform -- except Windows XP -- the system provides a way to decode H.264/MP4 video and Firefox hooks into that. But on Windows XP, this is not available.

What changed several months back was that Google stopped providing a an alternative format for the Flash player plugin as a fallback for users who couldn't play the other formats. That's how we ended up where we are.

I don't know how long you plan to stick with Windows XP. I'm only away of one complicated workaround for XP users, which is discussed and fine-tuned over many discussion pages here: Enable MP4 (H.264 + AAC) HTML5 video in Firefox on Windows XP without Flash. It doesn't work for everyone.

Firefox supports WebM/VP9 video on systems that don't support MP4/H.264, most commonly being Windows XP.

Microsoft provides a Media Feature Pack that provides the needed codecs to fully use the HTML5 Player in Firefox/SeaMonkey but only for Windows Vista and later. https://www.youtube.com/html5/

Another alternative is to dual boot with a Linux distro with a light desktop manager like Xfce. This way you have a system that is much more secure (compared to the old EOL WinXP) and can fully support the HTML5 player if you have FFmpeg packages installed. https://xubuntu.org/ is a option that should be light enough and is a distro that still supports the old 32-bit CPU's.

Unless the video was hidden for a while say for Patreon supporters early access, a newly uploaded video is going to be limited in video resolution options (say 360p at best is common) and such until it is fully processed.

Also if you have any Youtube related extensions installed you should deactivate or rather uninstall them. Many of such extensions have caused problems with playing videos on Youtube in last months.