I was surprised to see that Firefox 5 came out so quickly after Firefox 4. And then all went wrong. Not earth-shattering wrong, but Firefox began crashing and could not stay running for more than a few seconds. I immediately suspected an add-on. I found how to get Firefox started in Safe Mode (hold the Shift key and then click the icon on your taskbar or double-click the icon on your desktop). That allowed me to disable all add-ons. But even after doing that it was the same problem. So I downloaded a fresh copy from the net and re-installed the program. Same problem. ran through the safe mode start again, disabled all add-ons and then it was OK. I re-started on add-on at a time and stopped when I had turned all essential add-ons. I suspect the Forecastfox Weather but I have not narrowed it down to that point yet. More to come.
Update: Forecastfox was not it. More digging needed.
Update #2: None of the add-ons seem to be causing the problem now. Maybe the original install did nto run properly.
So far, the new version is fast, even faster than version 4. Nice! :-)