Firefox is also starting its third extension model now. At first it was XUL/XPCOM where extensions could basically hook into ANYTHING in the browser, then there was the Add-on SDK (formerly Jetpack), which is kind of nice, but a lot more limited, and now they're going with WebExtensions, because, well, Firefox basically does nothing but copy Chrome anymore ;)

But yes, that does make it complicated to figure out what's the best way to make stuff for Firefox.