I had a problem for a long time now that Firefox would go forward or back while I was trying to type into a text box. I discovered, somewhat by accident, the cause of this today. It turns out if you have a wheel mouse that uses detents to scroll in discrete increments, sometimes the wheel can stop between detents, depending on which mouse you have. A few seconds or minutes later, the mouse wheel will “pop” into place, normally just causing a little unintended scrolling. This happens with a lot of Microsoft Mice apparently.
But when you are holding the shift key, say because you are typing in a text field and capitalizing a letter, instead of scrolling, the wheel popping into place will cause you to navigate away from the page you are on, potentially losing all that you typed. It’s extremely frustrating, and it doesn’t happen often enough to realize what is causing it most of the time.
Here is the solution, for Linux and Windows at least:
Go to about:config and change mousewheel.withshiftkey.action to 0. This makes the wheel scroll even if shift is held down. This prevents firefox from losing text that you type with unintended navigation.
So if Firefox is going forward and back randomly while you try to type a message, this may very well be the cause.