Simply look under "about:support" tab for Multiprocess Windows to see if enabled or not. You can also force enable it in "about:config" by setting browser.tabs.

Simply look under "about:support" tab for Multiprocess Windows to see if enabled or not. You can also force enable it in "about:config" by setting browser.tabs.remote.autostart to TRUE if its not already (note this does circumvent having to disable non-compatible plugins such as Firebug).