I just recently realized the font icons I use in my Tumblr theme didn’t show up in Firefox. I then discovered this is because Firefox doesn’t let you host font files on external domains. For what it’s worth, the steps in this Pictonic blog post, "Use font icons on Tumblr", solved the problem. Just in case you have similar problems, or were reading my Tumblr on Firefox and wondering why random characters seemed scattered all over the page. Blame Firefox.