Those boxes appear when Firefox is using a font that doesn't have a character the site wants to display. Usually this is caused by overriding website fonts with a standard set of fonts.

You can check for and undo that change on the Options page:

"3-bar" menu button (or Tools menu) > Options > Content

Click the "Advanced" button and if the checkbox for "Allow pages to choose their own fonts, instead of my selections above" is empty, go ahead and check it. Then OK that change.

If you reload a problem site, do the missing character (possibly icons) display?

If not, there are some other things you might investigate related to downloadable fonts. As the next step, please check this setting:

(A) In a new tab, type or paste about:config in the address bar and press Return/Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste gfx and pause while the list is filtered

(C) If the gfx.downloadable_fonts.enabled is bolded and "user set" to false, double-click it to switch it back to true

If you changed that, you can reload the page and see whether that helps. Success?

Speaking of extensions, NoScript and uMatrix may block downloadable fonts from off-site. But I don't think other extensions have much effect on web fonts.

Hey thanks, I will try these the next time I see one. In retrospect, I should've bookmarked that last page I saw that had one. I'll be sure to post back my results.