Firefox 4 has made the browser faster and much more responsive, but its startup time can still be a bit on the slow end. According to Mozilla, the root problem is definitely extensions. In a new blog post, they stated that on average each add-on you install will increase startup time by 10%. So installing ten add-ons could double your browser’s startup time, assuming that none of them are real heavyweights.
They also released a list, based on automated performance testing, of the slowest performing add-ons. Apparently Firebug adds around 74% on to startup time and StumbleUpon 19%. (Unfortunately, it seems like a few of my favorite add-ons are on the list, the indispensable Firebug included.)
Mozilla plans to update their extension repository with warnings that display for add-ons that increase startup time by 25% or more.