I was not hijacking, merely pointing out that the file quoted was not necessarily the right one, and that the browser launched may differ.
cat ~/.local/share/applications/defaults.list
does not exist, either.
And does:
cat /etc/xdg/mimeapps.list
…exist?
It looked like a hijack since your comment wasn’t useful to finding the problem, and actually quite different. I recommend you rather start a new thread.
cat /etc/xdg/mimeapps.list
Not there, either. Sorry.
Thanks for your suggestions. I’ll stop now.
You don’t have to stop if you want more help, instead I think I’ll ask one of the @moderators to split this into a new topic for more exposure and so as not to polute the original thread.
MY last suggestion is checking if there is something in:
cat ~/.local/share/applications/mimeapps.list
or
cat ~/.local/share/applications/defaults.list
If there’s nothing, we can see in Thunderbird’s .desktop file if there are any weird associations. But to do that we need to know where the .desktop file is. Probable /usr/share/applications/ somewhere.
ls -l /usr/share/applications/ | grep thund
So, here is my output:
$ LC_ALL=C grep http ~/.config/mimeapps.list
x-scheme-handler/http=google-chrome.desktop
x-scheme-handler/https=google-chrome.desktop
I deleted the file, an after that:
LC_ALL=C grep http ~/.config/mimeapps.list
grep: /home/micha/.config/mimeapps.list: No such file or directory
and the problem is the same.
but after new preferred settings:
LC_ALL=C grep http ~/.config/mimeapps.list
x-scheme-handler/http=google-chrome.desktop
x-scheme-handler/https=google-chrome.desktop
x-scheme-handler/http=google-chrome.desktop;
It’s a shame that the discussion drifted a bit, my problem still exists. Do you have any other ideas for me as to what I could do? 
Try setting the default browser - we have a script for it [root tip] [Utility Script] Setting the default web-browser
Never click a link in an email - always verify the link target …
Never open attachments - always be suspicious with attachments …
Clicking links in an email is a high level security risk and a high level privacy risk - emails is often constructed in a way to reveal if the user has read it or opened it - e.g. web beacons and links are likely constructed with a unique identifier so spammers know if a given email is actively read and they are commonly used to spread malware.
Perhaps use Ctrl click - like in a terminal if a link is present or right click the link and choose open.
Done.
set-browser.sh google-chrome.desktop
Default browser set using google-chrome.desktop
But the problem still exists. 
Last thing I can think of:
Check if these files exist:
stat ~/.local/share/applications/mimeapps.list
and/or:
stat ~/.local/share/applications/defaults.list
Edit:
And if they do, post their content, please:
cat ~/.local/share/applications/mimeapps.list
and/or:
cat ~/.local/share/applications/defaults.list
LC_ALL=C stat ~/.local/share/applications/mimeapps.list
File: /home/micha/.local/share/applications/mimeapps.list
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: 259,2 Inode: 5507876 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 1000/ micha) Gid: ( 1000/ micha)
Access: 2024-02-04 19:03:26.612106773 +0100
Modify: 2024-02-04 19:03:26.612106773 +0100
Change: 2024-02-04 19:03:26.612106773 +0100
Birth: 2024-02-04 19:03:26.612106773 +0100
[micha@X1-Carbon ~]$ LC_ALL=C stat ~/.local/share/applications/defaults.list
stat: cannot statx '/home/micha/.local/share/applications/defaults.list': No such file or directory
Could the problem also lie here?
I always save the /.thunderbird folder on a USB stick and copy it to the home directory when I reinstall the operating system. Could it be that I created too many profile files? I have probably 20 different *.default directories in the /.thunderbird folder. The only question would be, which is the active profile that I am currently using? I could delete the others?
edit:
LC_ALL=C cat ~/.local/share/applications/mimeapps.list
[micha@X1-Carbon ~]$ LC_ALL=C cat ~/.local/share/applications/defaults.list
cat: /home/micha/.local/share/applications/defaults.list: No such file or directory
You could always check which one’s your default with:
thunderbird --profilemanaager
Or whatever the name of the executable is.
And then rename the other directories, which deletes them according to Thunderbird, and than test. If it doesn’t work, you can restore, if it does, you can delete it permanently.
I don’t think so, but
If you’ve finished, please post the output of:
locate '*thunderbird*desktop'
LC_ALL=C locate '*thunderbird*desktop'
bash: locate: Kommando nicht gefunden.
Edit:
LC_ALL=C locate '*thunderbird*desktop'
locate: can not stat () `/var/lib/mlocate/mlocate.db': No such file or directory
Incidentally, I deleted all unused profiles
Install the plocate package for that:
pamac install plocate
Update the database:
sudo updatedb
and try the command again:
locate '*thunderbird*desktop'
Done. But:
locate '*thunderbird*desktop'
[micha@X1-Carbon ~]$
OK, that was my last idea. I’m all out, sorry
Thank you very much for your efforts! 
Then I’ll leave it like that! and it’s supposed to be safer if I can’t open any links! 
I have now installed Evolution, now it works!