Does it help if you disable hardware acceleration?

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

In Firefox 4 Safe mode disables extensions and disables hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

cor-el,

Thanks for your reply.

In safe mode, the menu transparency problem went away. However, there is no update for my graphics display driver.

I do not get this problem in the menus of ANY other programs, so this must be a BUG in Firefox 4.

I note that, at this point, two other people have reported the same problem.

Please do what you can to see that this bug is addressed.