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.
UPDATE: I found that starting Firefox in safe-mode (Help, restart with addons disabled) turned checkboxes back to normal, but disabling them one-by-one did not have the same effect. So something in a setting somewhere in FF is doing this, I just have to narrow it down.
UDATE AND FIX: Shawn made a comment that pointed me to the fix. I went into /home/jw/.mozilla/firefox/xs0s2w1w.default-1385691302373/chrome/ and edited the file “userContent.css”. I put a “/*” at the very beginning and a “*/” at the very end to make it all into a comment. Saved it. Restarted Firefox and the checkboxes have returned. Note that your exact path will be different. Look for the hidden folder .mozilla in your home user folder (CTRL+H to toggle seeing or hiding hidden folders).