Firefox Always Thinks It's Running
For the last few days, iceweasel (Firefox on Debian) insisted it was running and refused to start. After much googling (and playing with other web browsers on my system, like Arora and Epiphany) it turns out that this is a known Firefox bug. Although now fixed in Firefox 14 (allegedly, I haven't tried) Debian wheezy is shipping a 10-something based iceweasel...
Anyway, this ends up reporting "Iceweasel is already running" without telling you how it figures that is true (from the above bug, it seems that not finding the profile directory is one such culprit!)
There are several workarounds in there, but mine didn't appear to be which is that somehow it became confused about where the profile was located. For me, this meant opening ~/.mozilla/firefox/profiles and changing the line in [Profile0] which says Path=abcd1234.default to the right path. This path can be found by looking at all the subdirectories in ~/.mozilla/firefox.