There is no way currently for Firefox to profile it's extensions without a significant overhead due to the way the Extensions API is built.
This extension will tell you a bit more about your startup times: About Startup
And Mozilla conducts regular tests on the addons every week. Here's the listing of the slowest addons (updated weekly)
editor's Note: it appears that Mozilla is no longer publishing this.
And if you want more info from the mozilla team, here you go and here.