Weird Firefox font rendering issue

[armstrca]

Some time within the last couple weeks, better errors stopped properly rendering some text for me in Firefox, but I have no such issues in Chrome.

Here's a screenshot of the problem. It's not legible, but if I copy and paste all of those weird characters elsewhere, they will render correctly as plaintext.

Which seems to be specifically linked to these settings in Firefox:

When I uncheck the "Allow pages to choose their own fonts, instead of your selections above" box, the page renders properly like so:

I recognize this may not really be a better_errors problem so much as a system configuration problem, but can anyone please advise on a fix for this?