Janek: I looked again at the Thunderbird problem, and I still could not reproduce it - however, I'd be quite surprised if the cause of your SSL problems would be changing the symlink. Thunderbird uses its own symlinks in /usr/lib/thunderbird/, and should not be affected by any change in /usr/lib/firefox/ (at least an strace of thunderbird shows that thunderbird does not use any libraries from /usr/lib/firefox/).
Janek: I looked again at the Thunderbird problem, and I still could not reproduce it - however, I'd be quite surprised if the cause of your SSL problems would be changing the symlink. Thunderbird uses its own symlinks in /usr/lib/ thunderbird/ , and should not be affected by any change in /usr/lib/firefox/ (at least an strace of thunderbird shows that thunderbird does not use any libraries from /usr/lib/firefox/).