Currently Suspend background tabs is a Firefox and SeaMonkey extension that will only suspend periodical actions happening in background tabs, these will only be resumed when the tab becomes active again. Lengthy downloads in background tabs are unaffected by the current version, as are videos or Flash. So some websites will still manage to make your browser slow despite being in background but that should be far less common now.
What's New in This Release: [ read full changelog ]
· Added "Don't suspend pinned app tabs" option
· Fixed error message appearing after opening View Source window