Currently Firefox seems to choose as its default font the last one that I select for a language on the Advanced popup. This can lead to anomalous results. If I set fonts for several languages, and the last one that I set is for a non-latin script and doesn't have glyphs for Basic Latin characters, English text without a specific language tag doesn't get displayed at all. This has led me to report errors on several web pages where the problem was actually in Firefox's settings.
Ons could argue that a font without glyphs for Basic Latin characters is an error, but plenty of them do it, and good software should be tolerant of errors.
To make things worse, if I change the default font, it also changes the font for the last language for which I set a font. I can't see any justification for doing that.
IMHO Firefox should allow users to set its default font explicitly and separately from any specific language settings.