(I'm assuming that you're accessing your BT email account via their website, rather than via a dedicated email client).
The first thing to try is accessing your email account via a different browser. (Much as I hate it, Internet Explorer is good enough for the job and bound to be on your computer - or, if you've got Windows 10, try Edge instead). If everything is normal there (as I suspect it will be), return to Firefox. Then . . .
If you've not got the Menu Bar (File, Edit, etc) across the top of your screen, put it there. (Right-click at the top of your screen and tick the relevant box).
Go to Tools > Add-ons > Extensions. Note down the names of any extensions you're using (and wish to retain), as you're about to remove them.
Then go to Help > Troubleshooting Information and click on Refresh Firefox.
Return to Tools > Add-ons > Extensions, search for, and then reinstall, your extensions.
Cross your fingers and try accessing your email account again.