With help of the just released "about:performance", I found several add-ons slowing down my Firefox.
After some additional investigation, it looks like there is a performance related issue in SDK, as a minimal SDK add-on
(each time you open or close a tab, it prints the tab's id in the console) created by Tab Reloader
's developer still performing bad.
More info and details:https://github.com/james-fray/tab-reloader/issues/3