Want to change up how you use Firefox? Some other browsers did the whole “tabs on the bottom” thing – and yes, Firefox can even pull off this trick with a little hacking around with the UserChrome.css file.

Here is what you need to add to your UserChrome.css file to get this to work:
/* Display the tabbar at the bottom */
#content > tabbox { -moz-box-direction: reverse; }
You can also move the tab bar to your left or right too, just [...]