Try downloading and installing manually through:

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

thanks ahnasr, i've tried that too with no luck. seem to be stuck.

I have the same problem. Also, when I pull down the Help menu in my Firefox browser, it says it's downloading Firefox 3.6.18...

I had asked the question also, but in a slightly different way, and I got this helpful response: In your current edition, click Tools, go to Add-Ons, then Extensions and remove "Ask Toolbar".

Close and then restart Firefox and go to Help | Check For Updates. The update should start automatically.

My solution was slightly different, but in the end, seeing what was going on in the Help menu in the browser did the trick. Turned out I had been stuck for who knows how long "downloading" a version 3! Also had a toolbar to disable but not the Ask.

My system seem to be frozen when trying to download the new version of Firefox. I tried your suggestion of removing ask toolbar, closing and restarting then updating but it didn't work. I've ended up with appx 30 downloaded 5.0.1 but it won't do anything automatically. Thanks! A true pain.