I-Firefox Isithangami Sokwesekwa

When I click on a link in Thunderbird to open in Firefox, I get the following error.

XML Parsing Error: undefined entity Location: chrome://browser/content/browser.xhtml Line Number 760, Column 13:

           <toolbarbutton id="UITourTooltipClose" class="close-icon"
</pre>
^
I am unable to resolve the issue. Is there a fix for this error?
Thanks in advance.
Bill

Hello.

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?