Recently I installed the latest edition of Firefox on my parents computer (they have Windows 7) and found that the new version was not only super slow but it would freeze up a lot and the interface was far too complicated and changed in comparison to the 4.0 release.
It's not often that I find a new version to be worse than the older one but this is a significant change that didn't par well with myself or my family. We love Firefox and want to experience it without a hassle.
Considering, I downgraded back to 3.6.16 and noticed that it's not only speedier; but even the tabs are more responsive. No more freezes and no more questioning "why does it fuck up when their is a video playing?"
Either way, I would encourage users to try this out for themselves. You will be shocked at the difference in speed and reliability. Sometimes, upgrading is not the best option. Don't worry however; support for 3.6.16 is going to be maintained for a very long time since many users will want to stick with the best version of Firefox. At least that is my humble opinion.
You can grab the older version here: http://www.mozilla.com/en-US/firefox/all-older.html