That's the beauty of XUL. Even a lousy web developer like me can tweak Mozilla, given a little digging around. Warning: this diary entry employs copious use of the <tt> tag.

dalke: Obviously the bug you quoted isn't resolved yet, so I don't know what the 'right' way to disable Ctrl-Q is (or rather, will be). However as a short term option, you can hack in a confirmation prompt to protect from inadvertant key combinations. This is, of course, a sub-optimal solution, since it'll get overwritten next time you upgrade Mozilla.

Short version:

Open up toolkit.jar and add this line (or something similar) to the top of goQuitApplication() in content/global/globalOverlay.js:

if (!confirm('Do you really want to exit?')) return false;

Long Version:

1. Find toolkit.jar in your copy of Mozilla's chrome directory.
   On Windows, typically C:\Program Files\mozilla.org\Mozilla\chrome
   On Unix (Well, Debian at least), /usr/lib/mozilla/chrome
2. Extract toolkit.jar into a temp dir. It's just zip file, so you can extract it with any zip tool.
3. Open content/global/globalOverlay.js and add the line mentioned in the short version to the top of goQuitApplication().
4. Re-create toolkit.jar with the changed file. Make sure the directory structure is preserved.
5. Start up Moz and try Ctrl-Q. You should get a JS confirmation prompt, and selecting Cancel should keep Mozilla open.