Hi harpthorn, glad to see you on Mozilla Support forum.

Try refreshing the browser:-

• Click the menu icon, then click on “Help”. Click on Firefox's main menu button, represented by three horizontal lines.
• Click “Troubleshooting Information”.
• Click on “Refresh Firefox”.
• Confirm.
• Click “Finish”.

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

Thanks, login ok now. Will now add my addons and hope it still keeps working.

Added McAfee WebAdvisor and ublock Origin and cannot login. Disabled ublock and still unable to login, disabled McAfee WebAdvisor and logins working. Will investigate further tomorrow.

Had a quick look on my laptop, it had McAfee WebAdvisor but not ublock. Added ublock Origin and it has stopped the logins working. I will continue tomorrow.

Hello, Maybe the add-ons are corrupted, it can be the reason because after refreshing the browser all addons removed and the login works perfectly, so there must be problem with your add-ons. Try Some other add-ons.

Hope it works !

Mcafee WebAdvisor is causing the problem. I have been using this for years so it may be a recent update to Firefox or WebAdvisor that has caused this. As I use Thunderbird for email (which still works) and only really login to bt for account reasons I can put up with disabling WebAdvisor when I need to do so. It would still be nice to know why my pc and laptop have both got this problem.

Just to say this was affecting a lot of people. B.T./McAfee have fixed the problem.