Most of the icons in Yahoo mail actually are characters in a downloadable font. If Firefox is not able to use that font, and can't find a good substitute, it shows the number of the missing character in a narrow box. The most common reason for this problem is if you set Firefox to use a standard set of fonts instead of letting websites choose their own fonts. You can review/modify that setting here: "3-bar" menu button (or Tools menu) > Options > Content Click the "Advanced" button, then make sure there is a checkbox for "Allow pages to choose their own fonts, instead of my selections above" If you changed that, OK the change and reload Yahoo (Ctrl+r) to see whether it loads the custom font. ''If it still doesn't appear:'' font downloads might be blocked, either by a setting or an add-on (e.g., NoScript). 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?