I hardly start up Firefox unless for upgrades. A list of add-ons that reduce loading / rendering performance would be even more interesting. I know by experience that the Skype one is very bad (and Firebug also slows down, but that's expected and luckily you can disable that when not debugging).
My Firefox 4 often became unresponsive for 3-4 seconds. Just today I finally nailed the cause - Mozilla's own Open Web Apps addon. After disabling it everything flies again :-)
So it's not only about startup time. My desktop computers (and so Firefox) usually run without restarts. I really hope that Electrolysis (out-of-process stuff, now implemented only for plugins like Flash) for addons will make finding bottlenecks far easier.
