You've updated to Firefox 4 for Mac and trying to run Google Page Speed in Firebug. But you get an error:
Unable to run Page Speed.
Your browser or operating system may not be supported. Please see Page Speed Compatibility for more information. Page Speed Copyright © 2010 Google Inc.
If you look at the Page Speed download page, you'll see that Mac OS X (x86 only), Firefox 3.6+, and Firebug 1.5.3+. So far, so good, right? Take a gander at the Page Speed Compatibility page and you'll find that "64-bit Firefox is not supported." Guess what? Firefox 4 is now 64-bit by default on Mac OS X. So until Google makes Page Speed 64-bit compatible, use the Finder to Get Info on Firefox and check "Open in 32-bit mode." Reopen Firefox, and Page Speed will be working again!