I have no issues with 2 of my providers. see screenshots What OS? What email site? Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search104

Did you clear your browser cache? https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox

Also try this option. Clear all items for a single site https://support.mozilla.org/en-US/kb/remove-websites-address-bar-suggestions#firefox:win11:fx100

It`s a extension. I did start in safe mode and was able to connect.

Which add-on caused the issue?

I would also recommend you run Firefox from Mozilla and sync the data kind of like a back-up. In case something goes boom. It's painful to see users come here who had no back up and the data on no other device. I don't even reply because it's game over. The regular version of Firefox failed to restore my session on 2 different devices. Because my data is in several versions and devices, I was able to send the tabs from another version. Just a warning. ;-)