With Firefox, I’ve noticed that as you make your browser window narrower and narrower, at some point the box you type in (i.e. the one I am typing in now) stays at a fixed width, meaning you cannot see the end of each line. While that is not an inconvenience on a proper computer, the limited width of my smartphone screen means that I can never see full lines when posting or replying.
The problem seems to be the width of the array of toolbox icons at the top of the window, which defines the minimum window width on Firefox. It works differently on MSIE, where the tool icons protrude outside the box on narrow screens - which looks silly, but is a bit more usuable.
From a usablity point of view, I suggest that the best solution for both browsers would be to wrap the icons onto a second line as the box becomes narrower. I do realise that might be difficulty from a programming point of view, but could you please at least consider making this change?