I'm trying hard to like Firefox 4, but the new browser keeps finding ways to annoy me. First, it moved the Reload button for no good reason (same for the Home button, but that's just as easily fixed). Second, it put the tabs at the top of the screen (again, easily fixed). Third, I'd swear it runs slower than Firefox 3.6, at least when it comes to starting up and loading pages.
All that's small potatoes. My real problem with Firefox 4 is incompatible add-ons (a.k.a. extensions), of which I encountered about a dozen after installing the update. Without the likes of PermaTabs Mod, Flstflip, and Craigslist Image Prefetcher, I might as well run Internet Explorer!
Fortunately, there's a way to turn off Firefox's compatibility check, which is what blocks these add-ons from running. A word of warning, however: while some or all of your "incompatible" add-ons may indeed run just fine in Firefox 4, there's a potential downside: one or more of them might wreak havoc, preventing the browser from running properly or even running at all. Try this at your own risk. (FWIW, I've encountered no problems on my system.)
1. Open a new tab, type about:config in the address bar, and press Enter.
2. Right-click anywhere in the main window, then click New, Boolean.
3. Copy and paste in the following preference name: extensions.checkCompatibility.4.0
4. Click OK, then set the value to False (which is the default).
5. Exit and restart Firefox.
Now you should have full access to all your add-ons (and plug-ins).
By the way, if you don't want to take the risk of messing up your browser, your only option is patience: most developers can and will update their add-ons to be compatible with Firefox 4, but you never know how long it will take.