Firefox update has caused a problem

I'm a newbie on this site so if this question has already been asked, then apologies.
I recently updated Firefox (April 12, 45.0.2) which has long been my preferred browser, and since then I am getting locked out of all sorts of sites I would normally visit. I can no longer play youtube music, just get a black screen that eventually stops on an error message. I can no longer access MSN games on line and some sites are coming up with a security warning that seems to be related to site certificates being incorrectly configured. On some of these, there is an option to allow, and some will open, but others just revert to a white window with an indexed list in blue, none of which help as they wont open.
I checked with my AV provider and they ran a diagnostic on their software, Eset security 9, ( updated this at the same time) and it all checks out. So the problem appears to be Firefox. I can get into all things with either IE or Chrome, but they're not preferred.
Anybody got a probable cause? I'm not particularly au fait with Win 7 BTW. I have tried uninstalling and reinstalling on May 6 which is now Firefox 46.0.1 but no change.

I have had the same in Waterfox since 45 for FF (it's an extension of it). They have changed some of the security and it's a royal pain. I have seen though that when you allow it now and future, then you will eventually resolve those issues. Sometimes they show at the top and sometimes bottom of the browser windows also. Anyways, don't suspect it's a Windows issue as it is the same way on FF on my iPad Pro. Might find help faster on the Mozilla boards or FireFox boards, although folks around here are pretty awesome:)

follow the directions here

rename the cert8.db file in the Firefox profile folder to cert8.db.old or delete the cert8.db file to remove intermediate certificates that Firefox has stored.
If that helped to solve the problem then you can remove the renamed cert8.db.old file. Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previous intermediate certificates. Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.



On a side note Ive not had problems on Palemoon or Waterfox when ive updated

AV's now include a safe browser site extention/security certificate check, its more than likely thats causing the problem. Check those first.

Roy
.

The only cert files i could find in the profile folder was either cert_overide which is a text document, or cert8 which is a database file while I suppose db stands for but it is not written as cert8.db is this the one you mean?

This is what it did to my attempt to open a you tube clip today.

Thanks Trashzone, yes I have tried that but without success. Will try it on a few more sites to see if it makes a difference. Am I right in thinking that s on the end of http means secure?