Firefox: substitute a font with another in any page

Firefox: substitute a font with another in any page

I'm using the stylish addon and it works great. However I would like to substitute a specific font - say Arial - with another of my choice every time it is found instead of setting a global font (which I don't like as I just want to substitute Arial) or using a stylish rule for every page. Is it possible to do this? Maybe with a greasemonkey script?