I have the same problem. I'm also unable to delete Firefox - when I try to delete, I get the same message about a previous install was interrupted and I need to reboot. I hate to say it, but I've reverted to using IE.

then Mozilla really need to do something otherwise they wont have anyone using it as no one will be able to use it.

I found a solution to the problem... I have searched online for all possible solutions Closest fix being this: http://support.mozilla.com/en-US/kb/The%20program%20must%20close%20to%20allow%20a%20previous%20installation%20attempt%20to%20complete

However still did not fix my problem and my computer keeps asking for reboot when i try to upgrade or uninstall firefox. By trial and error, I went to delete these 2 files: firefox.exe.moz-delete, firefox.exe.moz-upgrade and after that reinstall Firefox and works like new now!!! :)

Hope this helps.

It works.

Deleting the two files mentioned by 2mymisc above does work.

Great job