Site looks different in Firefox

Let me preface this by saying that I only started using WordPress about a week ago and this is the first theme I’ve used. Also, the last time I designed a website was maybe 10 year ago, so bear with my ignorance.

As of yesterday, my site looks different in Firefox. It looks just as it should in both Safari and Chrome, however. I think this happened after I uninstalled the WP Marketplace plugin.

Any idea how I can fix this? Specifically, the social media icons in the header and footer of the page aren’t appearing correctly. Also, on every page, there is a document-looking icon next to the title. There may be other things I haven’t found yet.

https://www.mindfulreboot.com

Help?