I've been using the Debian "alternatives" mechanism to support multiple installed versions of Firefox. Regardless of the specifics, the bottom line is that instead of
/usr/bin/firefox being a binary, it is now:
lrwxrwxrwx 1 root root 25 Aug 12 16:56 /usr/bin/firefox -> /etc/alternatives/firefox
lrwxrwxrwx 1 root root 21 Aug 13 17:07 /etc/alternatives/firefox -> /usr/bin/firefox.dpkg
This seems just fine and I can start Firefox from the command-line, or using an Alt+F2 launcher in my desktop environment etc. However, Thunderbird doesn't like it. When I now click web links in Thunderbird, I get a dialog box which says
Firefox is already running, but is not responding. To open a new window, you must first close the existing firefox process, or restart your system.
Surely that's not the case, right? What do I do?