Not a normal feature in Firefox.

Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• 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.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

Unfortunately, even when I restarted in safemode, pressing left control still opens up the bookmark menu. The button itself still works normally when i copy and paste, outside of firefox.

I turned on my computer this morning, and it went back to normal. I have no idea why it happened in the first place though

This can happen if key presses and releases aren't processed properly and in this case a key may have been in pressed state and the combination of the Shift key may have opened that menu. It usually helps to press the involved keys a few times.

Shift key? I used the control key extensively for a videogame after this problem, still had the problem with the control key, and it was only until i exited and restarted firefox did the issue go away