Web pages with a fixed width layout will always be a problem for any zoom feature. Assuming Firefox wants to adjust the layout so it can't exceed the width of the window, should it just do a proportional adjustment of widths or try to keep sidebars the same while reducing a main area, or wrap a sidebar to the bottom of the content (as though it were a floated element), etc., etc. Maybe the experience of programming zoom on mobile browsers will feed back some learning to desktop browsers, but UI designers have a long way to go there as well.

In the short run, maybe something like the Readability extension will help tame those unruly layouts?

It seems the Firefox source code and viewer model could in theory execute the ChangeFullZoom and ChangeTextSize functions independently in relation to the same content (in nsEventStateManager.cpp). Not sure there is any way to test that theory (e.g., using an extension).

Well, the best part about this idea is that all of the itty bitty details regarding how to zoom a page in and out have been already solved. Firefox already handles page zooming very well. All that would have to happen is exactly what firefox already does with ctrl+-, only with the text size constant.