I have been using Hide-Caption-Titlebar-Plus in FF for some time now and found it very helpful in increasing useful screen area for content instead of the toolbars and titlebar. Today with the update of FF to 36 it stopped working completely.

I can report, however, that there is a development version 2.9.3rc that you can get from the very bottom of the add-ons home page which does work very well, so far at least.
https://addons.mozilla.org/firefox/d...nux.xpi?src=ss