Maybe a question for the Mozilla bug tracker, but I’m trying here first.
I visited a website that showed a newsletter email where the font wasn’t supposed to be serif, and I noticed font-family was set as Helvetica without font-face, so it uses Helvetica system font (or a system fallback). I realised it was using the Deja Vu serif (which is my Firefox default). The reason I thought this was weird is because Arial on Firefox is matched to Liberation Sans (which is a lot more similar in style).
So I checked fc-match -s 'Helvetica' (a command I didn’t even know existed until ten minutes ago), and Liberation Sans is the first after Nimbus (which I don’t think I have), and sure enough, Chromium matches Helvetica correctly to Liberation Sans.
I think when system font matching (or fallback priority, in this case) is a thing, it should be respected by the browser. Deja Vu serif should only be used when no font-family is set at all, or if no suitable match is found. This is what Chromium does, and what most word processors do.
Anyone knows what goes on with Firefox here?