If you want to entirely switch back to the previous behavior, try set hidemenubar.floating = false.

That did it, excellent! Thanks for the rapid response and working solution.

With version 4.0.20101005 (in Ubuntu 10.04) there’s a black horizontal bar in place of the menu, whereas with the previous version there was none. I see all these hidemenubar.* options in config, can one of them be used to hide that ugly black bar entirely? I have no use for the hovering functionality, because I use a Personal Menu (https://addons.mozilla.org/en-US/firefox/addon/3895/#) button for bringing up the menus.