If you're excluding Ctrl + Mousewheel, then my best guess is to go to Tools -> Options -> Content, then click Advanced in the "Fonts & Colors" field, untick "Allow pages to choose...", then set the font and size above that check box to what you think would be a reasonable size. It's long-winded, but it completely circumvents usage of Ctrl.
There may be some more convenient way, but I seldom tinker with the Firefox settings pertaining to display options (I make sure to adjust anything and/or find addons right after I install Firefox, and I don't generally return and alter settings afterwards).