Many sites now use downloadable fonts to create their icons instead of traditional image files. So there are a couple Firefox settings that could be a factor, as well as the NoScript extension, if you use that (blocked objects).

(1) In the Advanced Font dialog, unchecking the box that lets the site use its own choice of fonts. To check that:

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

Here you need to "Allow pages to choose their own fonts"

(2) Disabling downloaded fonts in about:config. This is less likely, or you probably would remember, but here's how you check on it.

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

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

• If the gfx.downloadable_fonts.enabled preference is bolded and user set to false, double-click it to switch it back to true.

Does any of that seem relevant?

Thank you, it worked. It was the font. Now I can use Firefox on that site.