I had this exact problem. I fixed it by going down the the start menu, typing run and then entering "firefox.exe -ProfileManager" followed by creating a new profile and deleting the old one. Go and backup your bookmarks and such before you do so!

We don't do Start Menus or .exe here in OS X land. I downloaded and installed and now have a nice FF orb with an X through it. On attempting to open, get this:

  'the application "Firefox" cannot be launched. -10661'

OS X.4.11 Have mercy Mozilla dudes. I run my bidness through your browser.

Four new people reporting here with this problem since 9:30 EDT, US. Wonder what % of that is compared to the total.

This sounds like a different issue altogether.

What the heck? I try to update the fabulous Firefox 4 that so many rave about and now it won't even open. So is your goal to make Explorer the better option? I had nothing but good experiences with Firefox until I updated it, now I have to use Microsoft "Exploder" because Firefox is now a POS. Thanks too much!

Chimera, and all: Usually a FF update is a straightforward affair, requiring no backup of bookmarks or plugins. I've done it many times. FF 4 must be a different beast. The previous versions appear to have disappeared and I will not be surprised that everything I built into FF to make it useful have also disappeared.

We would all like to see someone from Mozilla respond to this. 71 out of 15 million downloads is easy to ignore for sure. But from a well satisfied many-year using experience with barely a blip to Nada is extremely discouraging.

Hello, Mozillians.... ?

I also had this issue after upgrading to Firefox 4.

I just downloaded firefox 3.6.16 installed it and Firefox opened up perfectly fine and everything was back to normal.

Thanks. Simplest answer the best. 3.6.16 is here:

  http://www.mozilla.com/en-US/firefox/all-older.html

Installed and started up, with all previous tabs, bookmarks, etc. in place as previously.

STILL need Mozilla to help on the bad FF 4 installs....

I fixed it by starting firefox in safe mode, then going to the Options menu, selecting the Advance tab and deselecting the "Use hardware accelarion when available" option.

There is no such option to 'run' programs in the Start Menu in Windows 7

There is no way obvious to start firefox in safe mode either in Windows 7

Anyone have any other bright ideas?

This happened with FF3 and now FF4. Im getting a bit tired of this software not being tested and bugs worked out before release.