One new feature of ie9 I like is that it alerts you to which add-ons are making your browser run slowly. Is there a way to get access to similar information in firefox?
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)
And if you want more info from the mozilla team, here you go.
[update] Added info in comment below: