Until this stuff is fixed for newer Firefox use this hack to get font size bigger:
create userChrome.css in the profiles /chrome folder and enter

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

menubar, menubutton, menulist, menu, menuitem, textbox,
toolbar, .tab-text, tree, tooltip, sidebarheader, statusbar {
font-size: 20px !important; }