Crazy amount of people trying to download it right now (including me, heh, sorry) so you might not be able to connect for a few hours. But, as long as you download it in the next 23 hours you'll be helping to get Firefox into the Guiness Book of World Records.
Just installed this myself it is noticeably faster I was suprised as I had 2 tweaked quite a bit. The only downside is my old adblock was incompatible and the sites to busy to get it again at the minute.
Your old version of Adblock is incompatible, but you can download a new version that is compatible:yes:
Edit: Oh I see your problem. I just had Firefox "search for extension updates" when I installed the new version, so I never had to worry about that. I recommend installing this extension as it's quite handy: http://www.longfocus.com/firefox/updatenotifier/