YSlow has been the tool for measuring page payload and providing the metrics you need to optimise web site performance beyond the web server. Shock horror when I recently upgraded Firefox to version 3.5 and couldn’t install YSlow.
Initial rumours confirmed the current version of YSlow doesn’t work with FF3.5 but also suggested development by Yahoo! had ceased. Meanwhile Google launched Page Speed—a likeable YSlow competitor (maybe?).
A recent posting from a Yahoo! dev would suggest YSlow will actually be updated but “the team has run into some integration issues with firebug 1.4.” If you want to install FF 3.x and 3.5 side-by-side, check out this guy’s article.