As a user, I might not change the config because:
* I don’t know there’s a config at all
* I know there’s a config, but I don’t know enough about fonts to make a good choice
* Most sites override my config anyways (unless I chose an even more aggressive config that breaks websites), so why would I care?
* The idea of some site specifying a font does not bother me, even if it’s a web font that will cause an extra 25kb of download.
For what it’s worth, I really like fonts and know my way around the Firefox config, but I honestly do not care enough to change the default font because of the last two items on that list.
Why not just let system designers choose the fonts their users are subjected to by default.
At least they seem to care more about accessibility than XYZ.coms marketing department.