Firefox 5 has been in Beta for quite a while, and the current iteration of Beta works just fine. It doesn't crash, it opens any site that we usually visit and Flash works for all of them.
This is not a big surprise considering that beta is supposed to come on June 24th and one big feature that was even requested by US Congress was the do not track option. To elaborate, Google and other advertising giants tend to look at what are you searching for, let’s say tablets or shoes, and they will serve you ads to match you interest.
Firefox 5 under privacy settings now you can check "Tell web sites I do not want to be tracked". If you prefer private browsing, a browsing that leaves no trace on your computer, there is an option Permanent private browsing mode, but this mode won't have any browsing history, in case you care about it.
This comes with Firefox 5.0, most likely before the end of this work week.