One of my all-time favorite Firefox tips is holding down the Ctrl key and using the mouse wheel to quickly zoom in or out.
Another all-time favorite: Opening a Web link in a new tab by clicking it with the mouse wheel. It's true! Give it a try. Go ahead, I'll wait.
Now, this may not work if you've installed your mouse maker's utility software and programmed the rodent to do something else when you click the wheel.
But the default behavior in Firefox (and, for that matter, Internet Explorer and Google Chrome) is to open the clicked link in a new tab.
Very handy! (Apologies to those of you who've known about this for years. But I guarantee you it's news to plenty of users!)