There are a few web pages out there for which Ctrl+ or CtrlWheel actually does change the font size and doesn't zoom anything else. I don't know why it's different for those pages, but I run across one every few months, and I'm always frustrated because the space allotted to the text doesn't get bigger and so there are only a few words on each line when I get the font to the size I want.

And you can make Firefox do that on every web page if you want, in version 17 anyway. You can switch between zoom mode and font size mode:
Alt > View > Zoom > Zoom Text Only (click on it to toggle it on or off)
Zoom Text Only works pretty nicely for sites like Mudcat that don't have tons of advertising with only a thin column for the text you're really interested in.