Do you have the problem where rectangles appear with two rows of two characters in them? This can occur when Firefox is unable to display a custom font used to show icons.

Could you check whether you are allowing sites to use their own fonts?

orange Firefox button (or Tools menu) > Options > Content > "Colors & Fonts" section > Advanced

After making that change, or after a site makes a change, you may need to dispose of previously cached files and existing cookies.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload Yahoo mail fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

If you use extensions (Firefox/Tools > Add-ons > Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure that such extensions aren't blocking content.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

It looks like it was AdBlock Plus that did it. Turned it off for the Yahoo Mail page and that fixed it.