Unfortunately, Firefox (at least the 3.6 version I experimented with) does not disable the command shortcut for Quit when you change it in System Preferences.
I think the quick and easy fix for this is to add ⌘Q as a global shortcut to something else innocuous. In my experiments, I found this to work:
- Open System Preferences.
- Click on the Keyboard pane.
- Click Keyboard Shortcuts tab.
- Click Exposé & Spaces shortcuts in the left side list.
- Under "Exposé" change the shortcut for "Desktop" to ⌘Q
Now when you accidentally hit ⌘Q, your desktop will magically and amusingly appear reminding you of your folly. Hit ⌘Q again and you're back exactly where you were.
As a sidenote, after a while you may want to change this back after you "unlearn" your behaviour. ⌘Q, the Mac equivalent to "ALT-F4" has been the standard shortcut since the introduction of the Macintosh, and like the other "standard" shortcuts ⌘W,⌘Z,⌘C,⌘V and so on, it's best to learn them to ease use of other Mac computers or when you upgrade.