You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I have ran several Malwarebytes scans on it, both in safe mode and in normal start up. The first few times I did find numerous PUP and PUF entries, but was finally able to eliminate them. The final two times I ran a scan, it came up with no malicious files found. I thought this would solve the issue but it does not seem to have done so. Do you suggest another malware scanner I should try? I have always had good luck with Malwarebytes.

FredMcD said

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

http://malwaretips.com/blogs/remove-sidecubes-virus/

"When the Programs and Features or the Uninstall a Program screen is displayed, scroll through the list of currently installed programs and uninstall SideCubes Toolbar, WPM17.8.0.3159, SupTab and any other recently installed unknown program from your computer."

Malware such as that may be installed under a variety of names or may be packeged with a variety of different programs, which makes it harder to locate and thus harder to remove.

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware#w_how-do-i-get-rid-of-malware Recommended 'tools' are listed in that support article, as hyperlinked by FredMcD in his firdt answer in this thread.