I have a completely fresh installation of Firefox 22.0 on Windows 7 and I just opened a website.
The website appeared to be zoomed (fonts as well as images) so I hit
It seems that this behavior is caused by the fact that from version 22 Firefox takes into account the font size set in the OS (e.g. "125%" corresponding to 120dpi). There is some info on Mozilla support forums:
http://support.mozilla.org/en-US/questions/962945
http://support.mozilla.org/en-US/questions/963211