Firefox has left me scratching my head a few times, I think they limit the amount of fallback fonts to resist fingerprinting. Long lists like these will get cut short. Text you intended to be monospace might not be rendered as such as the "monospace" at the very end was never reached.
I haven’t ever encountered that, and would expect the limit to be rather long (certainly no less than 32 elements, probably far longer) if there is such a limit. What there is is whitelisting, only exposing certain fonts in order to reduce font list fingerprinting: https://bugzilla.mozilla.org/show_bug.cgi?id=1121643.
