Does the line appear all the time or just inside Firefox? If the line is on everything including your operating system interface (Win, Mac, Linux, etc) then you may want to try updating your graphics drivers, if that doesn't help it may lie in the monitor or in the graphics processor itself. Sounds like your graphics processor is either getting old or is over heating. Which is a sign its about to burn out. If its only in Firefox then it maybe a plugin you have installed or a toolbar that is not installed properly. Need additional information about your hardware to really troubleshoot that.

This problem appears on the screen all the time. My computer knowledge is limited. I know nothing about graphics processors or whatever was mentioned in the reply. I appreciate the reply but I'm still baffled.

Do you have another monitor you can try? Or can you change screen resolution to a larger output?

You may have opened the sidebar.

Is anything checked in View > Toolbars > Sidebar?

See also:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

No other monitor. I'll probably have to tote the iMac (heavy!) to a nearby Apple store to consult the genius bar. But thanks anyway. Cliffotf Browder