After following these clear instructions for upgrading to Firefox 4.0 on Ubuntu 10.04/10.10, after delighting in the speed and reveling in the speed and other good stuff, I wanted to get firebug back so I could do some socially useful work!
So I went to Tools / Add Ons from the browser menu, and a pop-up window showed me all my disabled stuff that was incompatible with Firefox 4.0
After finding out that 1.7 was the Firebug version that was indeed compatible with the new Firefox 4.0, downloaded firebug-1.7.0.xpi from this page: http://getfirebug.com/releases/firebug/1.7/.
Then, I returned to Add-ons Manager at Tools / Add-ons and from the little drop down list icon to the left of the search box in the top-right corner, I selected Install Add-on from File…
I was told Firebug would be installed after restart, and it was. After enabling the Add-on bar from View / Toolbars, there was my beloved little bug in the bottom right hand corner where it belonged. A screamingly fast firebug for the screamingly fast Firefox 4.0!
And that’s how I got my Firebug back! Lots of new features for 1.7.1 also!