With Firebug installed, you most likely have JIT disabled http://antennasoft.net/robcee/2009/12/15/firebug-and-the-jit.... I would suggest you use a separate 'webdev' profile for such things (the inbuilt web console that is now in firefox is also getting there).
That was true in 2009 but was fixed in January 2010: https://bugzilla.mozilla.org/show_bug.cgi?id=534120#c52 (although it’s true that if you’re actively debugging scripts the JIT turns off - Firebug has the script panel off by default)
Incredible, I've been doing all my browsing for the past year with the script and console panels activated and had no idea that it was disabling the JIT for every page that I visited. Just turned them off, and I can't believe how accustomed I'd become to sluggish webpages. Thanks for the link!
