Since the most recent update, Firefox won't load some pages. The same pages load without any problem in Chrome. I'm running 3.6.3.
Flash could be it. It happened on TypePad.com and this URL when I tried to sign in. Also, Joe's Goals when I tried to sign in. No screen shot to send, since there was nothing on the screen. The page was blank, and I had the spinning wheel in the Firefox tab. Interestingly, it didn't time out.
In that case, it's most likely an upstream issue with Firefox. Check to see if there is a submitted bug report of this issue on a Mozilla channel. If you can't find one, you may have found a new bug. As far as I know, the next update to Firefox will be 3.6.4 and will be a substantial update. So it will probably be fixed with the next release, anyway.