Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
  

Don't select anything right now, just use "Continue in SafeMode."

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

Thanks. Safe mode did not bring in the icons. Is there a file of icons somewhere, and gmail just takes advantage of them? I have commented to google and am totally ignored, but surely I am not the only Firefox user on gmail. Do others have this issue or not??? Do you have a suggestion that does not involve chrome, IE, or updating an older computer to Win7?

While I was attempting to use Ad block to remove the ads at the top of the new Gmail GUI, playing around I accidentally *un*clicked in Firefox>Tools>Options>Content on "Load Images Automatically" box.

Apparently, it's not possible to "disappear" or "hide" the ad bar at the top of the new Gmail interface... and it was totally frustrating to see across the board images, icons, etc., were missing at first, and I couldn't understand why (what on earth makes me investigate or "make significant changes to" my computer when I'm really supposed be going to bed and I'm WAY too tired to pay attention to what I'm doing ... must... keep... hands ... off!).

But check out if this box in Firefox is checked or not. Once I turned it back on the icons and images re-appeared.

The ads are still there, but, I guess as long as they don't start to take up half the screen before I'm allowed to view my emails, for now at least I can live with this!

Good luck! Jesster

Thanks Jester... I had the box checked, but with one exception. It started with "ssl. ??? static" but I did not write it down. A left over from the unsuccessful adblock adventure. I removed the exception, and restarted firefox. Now they appear. Many thanks.