I had the same (or at least very similar) problem.
In my case, I installed "UserAgrent Switcher" to "disguise" my firefox once.
I believe that this extension made something inside the browser stuck in loop-y pothole, and the warning message that you mentioned above pops up, and Firefox is a no-show.
I believe firefox kinda fooled (or identified) itself as it was MSIE while calling for some functions internally or vice versa.
(FireFox self-hypno-ed itself to be MSIE! A case of identity crisis? ^_^)
In a nutshel, enter the following line in [prefs.js].
I have used many versions of mini-Mo, from Firebird to Firefox, and many upgrades in between them so far.
I also have Mozilla, and Netscape browser.
As a result, I have many js/installation/Profs files that are here, there, and everywhere.
Don't delete Java.
There is no reason to delete Java directory as mentioned above.
(You have to reinstall JAVA again if you do)
Don't Rename or delete Mozilla directory.
It may reinitialized your extensions, settings, prefs, profiles, and/or destablized the program entirely.
Always make backup of any file you want to edit BEFORE you change anything in it.
All you have to do, given that your problem is exactly same as mine, is locate current [prefs.js] file and enter the following line at the end of it.
This will override browser identity crisis.
Don't forget to change back your useragent (AKA Browser ID) back to default once you have firefox on.