Sounds like an extension might be getting in the way? try disabling all extensions and relaunching FF with multiple tabs open

Thank you for replying. I don't seem to have any extensions. Additionally, Firefox doesn't remember my home page either, even though that is also specified correctly.

I have had similar symptoms with some malware that was installed on a client's computer. Try to scan the computer with anti-malware software and see if the problem persist..

Looks like I needed a tune-up to fix this and other issues. Many thanks for your replies!