The screenshot didn't attach. Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (200 KB) and do not click Preview after you have attached the image.

It sounds like this issue: This can be caused by the Helvetica or Geneva bitmap font or another (bitmap) font that can't be displayed by Firefox in that font size. Firefox can't display that font in the specified size and displays gibberish instead. You can test that by zooming out (View > Zoom > Zoom Out, Ctrl -) to make the text smaller.

Uninstall (remove) all variants of that not working font to make Firefox use another font or find a True type version of that font that works better.

Okay, so I can use Helvetica in my various programs, but I don't see it in my font folder. Any ideas of where it could be hiding? Am I crazy or shouldn't it be in there?

By the way, thanks for the answer -- very helpful.