Anyone seeing slowness in Firefox 4.0 compared to earlier builds, could I ask you again to try these about:config changes and report the results? This was one of the only things we changed between 2011-03-05 and the final release, and it might affect memory usage and speed:

toolkit.browser.cacheRatioHeight -> 2000
toolkit.browser.cacheRatioWidth -> 1200

After changing these prefs, please restart the browser.

This should reduce the memory usage (but might increase checkerboarding). Please let me know if this gives you a better experience. Thanks for your help!