Here’s a site worth bookmarking. Add-ons are the big thing Firefox offers that the other browsers don’t, but it sometimes comes at the price of performance. And I guess Mozilla is tired of that, so now they’re testing add-on speed and publishing the results at https://addons.mozilla.org/en-US/firefox/performance/ .
I don’t see anything I use in the top 10, but Adblock Plus, unfortunately, is #12. I’ve been recommending it as a security measure for a very long time. Of course, antivirus software slows your machine down too; security is rarely free, in terms of computing resources. On a marginal machine, you might want to take another approach to blocking malware domains.
Greasemonkey is #15. I may be though with using that; I didn’t reinstall it when I upgraded to Firefox 4 and I find I’m not missing it. There are some web sites that work a lot better with Greasemonkey customization, but I don’t visit many of those anymore. Which makes sense. I have little use for sites that care so little about usable design as to leave it to their users. (I’m looking at you, Reddit.)
And at #49, I find Better Privacy, but its performance impact is only 1%. That’s another thing I haven’t reinstalled since upgrading to Firefox 4, but I probably should.