I'm new to Tor, and would like to ask if three 'cosmetic' Firefox addons - Tab Mix Plus, All-in-one Sidebar, and Scrapbook - pose an anonymity threat?
You should not need to use any addons with Tor. If you want addons then Tor is not the browser for you. Personally I would never risk Tor's anonymity by using 3rd party addons. That's what Firefox is for. Not Tor