Do a malware check with some malware scan programs. You need to scan with all programs because each program detects different malware. Make sure that you update each program to get the latest version of the database before doing a scan. * http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware * http://www.superantispyware.com/ - SuperAntispyware * http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page * http://www.safer-networking.org/en/index.html - Spybot Search & Destroy * http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and Searches are redirected to another site * also Firefox 10 - getfirefox.com

There is probably nothing wrong. It may be caused by Firefox switching on some display buffer that isn't cleared and that can show as all kind of colors. I've seen post where the color red is mentioned. I assume that Firefox is busy doing something else and that makes it appear.