Hmm, that list area is translucent for me. If you switch Gmail back to the basic theme (solid white background behind the list), how do the gray patches change, if at all?

Could you test in Firefox's Troubleshoot Mode? In that mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help assess whether one of those, or some combination of those, is causing the problem. Here's how.

If Firefox is running:

You can restart Firefox in Troubleshoot Mode using either:

• "3-bar" menu button > Help > Troubleshoot Mode...
• (menu bar) Help menu > Troubleshoot Mode...

and OK the restart. A small dialog should appear. Click the Open button.

If Firefox is not running:

Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) A small dialog should appear. Click the Open button.

Note: Don't use the Refresh without first reviewing this article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Any improvement?

Ref. Diagnose Firefox issues using Troubleshoot Mode

I tried changing my settings in gmail but it did not work. I opened firefox in Troubleshoot mode and it did not have the problem. I disabled all extensions and reloaded firefox. It's still doing it. Any other suggestions? Would plugins cause the problem?