Link in email (Thunderbird) won't open in Firefox
I am using Ubuntu 21.04 and after the upgrade I got different issues with Mozilla products. FF is: 88.0 64-bit TB is: 78.8.1 (64-Bit)
One is: "Firefox is already running, but is not responding. To use Firefox, you must first close the existing Firefox process, restart your device, or use a different profile." appears if I click on links in emails. It doesn't matter if they're linked to youtube, are an online version of the email etc. Always the same message.
I tried to assign Firefox as a standard browser and changed to another one, started the system again, ran Thunderbird and Firefox and it still happened. I tried changing something in the config editor, doesn't help at all. I checked for files / attachments and what should happen with http/https. There's no entry, which is fine according to mozilla help. The settings are the same as before the upgrade. I don't understand why it doesn't work anymore.
And second problem is: When clicking a email link in Firefox it doesn't open in Thunderbird. Error message is: "Thunderbird is already running, but is not responding. To use Thunderbird, you must first close the existing Thunderbird process, restart your device, or use a different profile."
It also doesn't work if I have run FF and TB in safe mode at the same time or in other constellations (FF in safe mode, TB not and vice versa)
Any other ideas?
What I did so far, since I posted here and asked for help: -I de-installed FF, and re-installed it. -I deleted cache for both TB and FF. -I renamed .thunderbird in thunderbird.bak, deleted cache again and started TB. I had a fresh Thunderbird then, but didn't help. conclusion: it is Firefox that causes both errors. -I followed the instructions in the mozilla where similar problems are explained. But the solution there didn't help me at all.
Any ideas what I can do or try is more then welcome. The problems occurred first after the upgrade to 21.04 and I haven't changed anything in addons/plugins/settings etc.