Do you have that problem when running in the Firefox SafeMode?
A troubleshooting mode.
You can open the Firefox 4.0 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut. Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut to open it again.

If not, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Thanks, the-edmeister, for your reply. Those menus do work in safe mode. However, I have a lot of customized settings, so I don't want to use SafeMode all the time.

You need to figure out which add-on is causing that problem, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Thanks! Actually, those menus started working now - I don't know what fixed it. I haven't restarted the computer (although that would perhaps have been an alternative), just put it in sleep mode. Anyway, I'm sure the link above would have been very useful otherwise, or if this problem returns.