Since I wrote this, the client was able to log in normally in Safe Mode. So he reloaded Firefox but still gets the same problem in normal Firefox. Is there any one out there who could shed some light on this?

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

See:

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

Thanks, this did the trick. It was an add-on that we now disabled.

i reaaly appreciate it.