This very tiny (15KB) add-on will add a "Restart" menu item to the "File" menu, optional toolbar button, and a ctrl/cmd + alt + r hotkey to Firefox, Seamonkey, and Thunderbird which will restart the program. It can also be installed/uninstalled/enabled/disabled all without requiring a restart, hence the name "Restartless Restart".
Functionality is different for Fennec, to restart Fennec simply enable Restartless Restart, it will always be disabled.
Restartless Restart is similar to the QuickRestart
add-on except it doesn't work on FF < 4, it does work on FF 4, it's restartless, it's 12KB instead of 34KB, it works for the latest versions of Fennec, Seamonkey and Thunderbird.