First thing that we need to do is create shell script:
exec /path/to/firefox "$url"
Save that script to Thunderbird default folder. In my case: ~/.mozilla-thunderbird
Now, make that script executable.
Please close Thunderbird and open file prefs.js in your favorite text editor. You can find that file inside Thunderbird profile directory. For example:
Now you need add following strings to the end of prefs.js
Where openlink.sh file shell script that we saved before.
Now just start Thunderbird and click to any link. It should always open it in Firefox.
I don’t know why Thunderbird didn’t use system default browser either i dont know why there is no settings about broswer in Thunderbird Preferences. I hope this will be fixed sometime, because way above is not clear and not portable. Usually same methods called “hack”