I am on Firefox 54 on Ubuntu and just noticed that any implementation of the HTML checkbox shows up as a tiny dot instead of a checkbox. Chromium browser is normal.
I first noticed when I went to check my settings in FBP (Facebook Purity add-in for Firefox). But I confirmed it on an HTML test site:
Firefox looks like this:
Chromium looks like this:
I tried searching for a solution, but only got a zillion unrelated results.