Just ran into a problem with Firefox. The last few days I haven’t been able to launch Firefox. I would attempt to launch the app, the hard drive would crunch for a few seconds, then nothing. No error or anything. Firefox would never start.
So today after finally going though tabbed browsing withdraw, I couldn’t take IE anymore, so I dug into the problem.
Using the free handy filemon tool by Sysinternals I specified a filter to see what files firefox.exe was accessing. It happened to be that the last file it opens was my history.dat file. So, I deleted it and tried to start Firefox again. It worked!
The history.dat file is located in your profile folder.
Doing a little more research, the problem is likely related to this bug that has gotten a lot of attention:
https://bugzilla.mozilla.org/show_bug.cgi?id=319004
So, basically the filemon tool came in really handy. If I didn’t use it, I probably wouldn’t have found the solution very quickly. You get a lot of hits in Google for “Firefox doesn’t start” 
Pingback: Feral Gifter » Firefox Loading Problem