Something strange is happening in Firefox 41; I tested in Windows.

Usually if I uncheck the box for allowing sites to choose fonts and OK that, the change is immediate, and if I reverse that process, it also is immediate. But when I tested it just now with some unattractive fonts, it took a couple tries to switch it, and then I got stuck with the wrong ones for several minutes, even when I confirmed using about:config that browser.display.use_document_fonts was set to its default value of 1. Even after exiting Firefox and starting it up again. After using the dialog several times, switching between Latin and Other Writing Systems, checking, and unchecking, and rechecking, I finally was able to escape Algerian/Comic Sans hell.

That probably doesn't help you very much if at all. I think this functionality is still in there, but there's some disconnect in how/when the settings are applied that I don't understand.

I dislike serif fonts on-screen -- too "busy" for my eyes; so, I use sans-serif fonts. The workaround, here, was to set a sans-serif font as my choice for a the serif, and make the default font "serif" (see below)

Firefox 41 will allow icon fonts to work with the font setting in "Options/Preferences > Content > Colors" disabled.

• http://www.ghacks.net/2015/07/21/firefox-41-mozilla-modifies-how-icon-fonts-are-handled-by-the-browser/