This problem had been a nuisance for me for a while,  In the past, whenever I exited out of a window with lots of tabs, I’d get a nice warning message asking to confirm that I wanted to exit out of all pages.

This was fantastic because, on a qwerty keyboard, Q and W are right next to eachother.  So, one slip of the finger turned a command to close a single tab into a command to close the whole darn window.  But it seems that new versions of  Firefox have disabled this warning by default.  Today, with the help of Mardeg from the #firefox channel on irc.mozilla.org, I re-enabled it!

Here’s how:

1. Navigate to about:config?filter=browser.showQuitWarning in Firefox.
2. Then you will be confronted with some warning about voiding your warranty (see below)  You really needn’t worry about this for the setting we’re changing.  The warning applies to all navigations to about:config as some settings in there can indeed be precarious to change. This one is not.
3. Next you will be faced with the setting itself.    Simply double click on it or right click to toggle it as true.
4. Finally close out of the tab and you’re done!  The next time you exit out of firefox with multiple tabs, you will receive a warning.