Update the video display driver to the latest version.

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

• Don't make any changes on the Safe mode start window.

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

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

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

It was the hardware acceleration. -Thank You

This problem should not be because of hardware acceleration and I think I found the answer. I was getting odd graphical iisues in Firefox 4. Not just with the strange vertical lines. By disabling hardware accelration it did indeed fix the issue with the lines underneath where I typed in boxes. But the other problems were still an issue. I tried to fix the other issue by disabling all my addons and it did not make any difference. BUT when I restarted Firefox 4 in safe mode it did. Which led me to believe something else was at play here.

I remembered the UserChrome.css I had in my Firefox profile folder. I renamed that file and restarted Firefox. All my graphical bugs vanished. Out of curiousity I also enabled hardware acceleration. Voila! No more vertical lines either!

Now I did a reinstall of Firefox 4 just to check if it created a UserChrome.cc file but it did not. So I can only assume that there is something in that old file that others have when upgrading from Firefox 3 that causes this issue and other graphic probs.

I advise anyone who noticed this issue to try renaming UserChrome.css if it exists and see if it resolves the issue or not.

Can you post the content of that file userChrome.css using code tags (<pre> </pre>)?