> And if someone can trick me into modifying /usr/lib, they can just trick me into replacing Firefox completly with their malwared build, so signing will not save me anyway.

As you said yourself, that's a much bigger hassle and cost. In other words, it's an effective deterrent. Writing to a user owned file is a very low bar for allowing privileged code execution in the browser.

A long time ago browsers used to be infested with all kinds of toolbars and extensions automatically installed by third party software, I for one am glad to not have to worry about that in my computer and on networks I manage or frequent.