On Linux in specific and Unix more generally, there is a notion of default system fonts that are known by the magic names of 'serif', 'sans-serif', and 'monospace'. What these magic names map to is determined by a complicated pile of decisions that are spread around the system in the form of Fontconfig (also). These decisions are generally fairly opaque and hard to peer into unless you're relatively experienced with command line tools.

Unless you configure it otherwise through Preferences, Firefox normally uses these standard font names if a website doesn't set specific fonts (or uses the generic names, which often show up as the last fallback in CSS). This means that Firefox is exposed to mysterious changes in what your Linux system actually maps these fonts to, which can vary. However, I discovered today that Firefox has a very convenient feature here.

If you go to the general section of Preferences and haven't customized your font choices, you will probably see the "Default font" listed as something like "Default (DejaVu Serif)". What this means is that Firefox is using the default meaning of 'serif', but is also telling you what font it actually maps to. If you go into "Advanced..." and have not customized your font choices, you can see what all three of the magic names map to. This can be very handy for sorting out strange font issues.

(This doesn't quite give you complete details, as it omits the Fontconfig style, which can vary and theoretically may make a difference.)

Firefox's Web Developer tools will give you a version of this for any web page (including ones that don't set any CSS fonts and so are using the system ones). However, as far as I can see the "fonts" pane won't tell you exactly why 'DejaVu Sans' is being used, just that it is (and that it's a system font). I also don't know if this will peer through other Fontconfig font aliases to tell you what they map to, or if it will faithfully report them under their pre-alias names.

(Because I was curious I just checked and Chrome does not show this information in its font preferences; it just tells you you're using the generic names. Chrome also appears to give less information about fonts in its Web Developer tools, but I probably don't know what I'm doing there.)