So, I checked it out: Two machines are working (one with 0.7.9.4 another with the actual beta 0.8.3.1). The one with the problems (a 0.7.9.4) is the machine I detected the bug with for the former versions.
Looks like the prefs of this setup got confused with the buggy firefox version. Creating a new setup (by removing ~/.mozilla/firefox) was working fine again.
