There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Thanks, I tried all those links suggested including testing Bitdefender and proxy settings (BYW I am not tech proficient). SKY advice on SKY help forum suggesting reporting for Error 288 / access denied phone SKY. Did that may be that IP address has been blocked somehow. Thinking back in the past this has a familiar ring to it, whereby updates / tweaks at "central" disrupt one's settings. OS is W10 Pro V 20H2 FF is 89.0.1 (64 bit)

Anyhow contacted SKY again, through to tech, gave IP address as recommended for a- in their hands now. But to reiterate all other website bookmarks work, and SKY works with Microsoft Edge, which rather suggests it is a specific issue with FF, not SKY for Error 288 / access denied. But I can be well wrong.

There are times I hate computers. FWIW was weaned on a BBC B - pre-internet

Hmm, since another browser works, it would be odd if the issue were your IP address, unless there is a difference in the way they connect, different proxy or VPN.

You can check the proxy setting here:

• Windows: "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type proxy and Firefox should filter to the "Settings" button, which you can click.

Try "No proxy".

Any difference?

Thanks I did try no proxy - no change. I have repeated as per your advice and restarted - no result.

The official advice from SKY is at https://www.sky.com/help/articles/problems-accessing-sky-yahoo-mail

viz. Error 288 or Access Denied

When you sign into your Sky email, you might get:

   Error 228 - To fix this, you'll need to contact us and tell us your public IP address.
   Access Denied - To fix this, you'll need to contact us and tell us your public IP address, and the full reference number that's displayed on the error page.

To contact us, select Yes to the question at the bottom of the page.

I have done this which is about the public IP address (home PC). They suggested give 72 hours to see if they can fix. If my memory serves this happened a number of years back. Interestingly if one firtles for the question on line on e.g. SKY forum - this is happening to some, not all, users, and has been for some time over several years.

I can't comprehend why one website bookmark, on one browser, for unconnected individuals, over time, when everything else if fine crashes. SKY did suggest when I phoned that somehow one gets locked out. But why only Firefox? Works with Edge and e-m client, otherwise I wouldn't have got the two responses so far.

All I can say is that in the past I have had an issue with Thunderbird and Avira A/V, but checking solutions with Bitdefender already has made no difference.

All rather bizarre - one website bookmark, one browser, one person but known issue since 2018.

Sometimes I just want to give up...................................

The typical Access Denied screen has a line starting with Ref. followed by a long number. That one is characteristic of a web application firewall (WAF) operated a security service provider on the company's behalf. It may be that the WAF provider didn't like the requests generated by Firefox -- viewed them as bot-like due to incomplete or modified data -- compared with Edge. The involvement of a separate vendor also would explain the delay in resolving the issue.

Indeed seems logical, hence I will have to wait for SKY to "unblock" / allow access. I suppose one frustration is that all was tickety-boo until a Firefox update a few weeks back, however, this issue on our second PC a/c predates this update and the issue on my a/c side has only been the past few days - after the FF update (pretty rounded tab edges!). So in the absence of a solution I will have to wait to see if SKY tech can find / fix at their end. The other odd thing on perusing fora and others with this issue dating back to 2018 is how random it all is and that one has to spend much time breach the SKY defence algorithm to get to contact a human !

OK someone on SKY community forum has solved this. https://helpforum.sky.com/t5/Broadband/SKY-access-denied-on-Firefox/m-p/3715120/emcs_t/S2h8ZW1haWx8ZGlnZXN0X25vdGlmaWNhdGlvbnxLUTdJMkVQMUhEM0JWUHwtMXxPVEhFUlN8aEs#M257252 The standard links by e.g. Google go to the SKY homepage and if one signs in there, then access denied. However, if one manually types in the exact letters https://skyid.sky.com/signin into the URL box (this appears as a link here which will take you to the SKY homepage, so just type in the letters exactly and manually in the URL box, do not use what is a link here) then it goes direct to sign in. This page is subtly different. Old page - sign in top right. "New" page - sign in top mid left and different appearance. Hence different pages. I have tried using this as a bookmark x3, restarted, redone it x7 - so it works. The inference is that the SKY homepage link as per e.g. Google search (SKY Homepage as appears on a bookmark) on Firefox is broken. Whereas SKY sign in (appears as SKY sign in on bookmark) as above works.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.

update:

still working OK on my a/c. The second a/c works but once you put in the URL letters https://sky.id.sky.com/signin and reach log in - bookmark it then, before actually logging in. Then sign in. Otherwise if you sign in and then bookmark, it reverts to SKY homepage in which case you get SKY homepage and "access denied".

So.

1. delete old bookmark and cache history etc

2. type in the exact letters as above in URL box, (do NOT click what has become an automatic https link above.)

3. then arrow at URL box.

4. You reach signin page.

5. Do not sign in yet, but bookmark that page.

6. Then sign in.

7. On second a/c the page reverted to the old style, but the bookmark does work properly on several checks.