Updating to new versions of Firefox can be a pain. If it's a brand new beta there probably isn't a mouse gestures addon that is compatible yet (Firefox 4 beta was without a mouse gestures plugin for months). Firebug usually has a working version, but don't count on it. Luckily, there's usually an easy fix.
Download the nightly tester tools addon, restart Firefox like normal, etc. Then when you upgrade to a new Firefox version, if it creates an incompatibility with an addon, see if forcing the compatibility will work. It might not, but usually it will. Unless there were API changes in the browser upgrade, Firefox won't know any difference. Especially since Mozilla has now implemented a six week release cycle, there is much less chance that there will actually be a real incompatibility with addons, other than its config file saying that it's meant for the older version.
Seems silly that we need to do this, but at least there's a work-around.