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Upgraded to FF3.5.x: initially, only problem was several extensions (Tab Mix Plus, Personal Menu) wouldn't work. Rebooted/restarted, but not only didn't help, FF itself wouldn't run (at all). Uninstalled/reinstalled both FF 3.0.x and 3.5.x, rebooted PC -- no joy. Noticed that with Task Manager open, trying to boot FF was followed by a brief (2-3 seconds) appearance of process "firefox.exe" but promptly disappeared. Running FF Safe Mode didn't work; rebooted Windows in Safe Mode, but FF still wouldn't launch. Was able to start Profile Manager, but button for New Profile didn't function.

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Did you try installing 3.5 directly (not an upgrade from 3.0)? Did you try FF Portable ( I have no idea what the problem is; I did the same things with the same extension problems, but it worked fine. – Daniel H Aug 6 '09 at 4:58

Try a fresh install in a different directory.

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Your installation seems to be corrupted.
Since you have already tried an uninstall and reinstall once,
Suggest you try a cleanup of the install with something like RevoUninstaller.

Then, do a fresh install.

Checking that a portable edition works fine is also a good idea.

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