Okay, so I promise I searched online before posting here, and I couldn't find anything. And after posting this, I found it elsewhere right away. My problem is solved. For anyone else who has this problem, they should go to the Firefox Menu, and go into "Add-ons". Then, go into "Extensions" and remove any that you don't need. I removed three "Converter, "Valance" and "ADB Helper". The problem went away!

Good job   ! But you should do a little more than just that, cause   undefined.com   is a browser hijacker and may have infected your computer with a virus/malware.

Go to the 3-bar menu => Addons => and look for any unfamiliar or suspicious looking extensions, like e.g.: SafeSearch Incognito SearchAssist Incognito When you find any :   remove them. So, don't just remove extensions that you don't need, even if the problem seems to be solved now.

Do the same in Windows' Control Panel => Programs.

However :   most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article :
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners.
They all work differently   -   what one program doesn't pick up, the other might.

Also see : https://malwaretips.com/blogs/remove-undefined-com-redirect/

The most likely culprit is probably Converter (@Converter).

The other two extensions you mentioned are special extensions that come with Firefox as you can see by the trailing @mozilla.org (adbhelper@mozilla.org and (xdevtools-adapters@mozilla.org).

ADB Helper and Valence are special Devtools extensions as you can see if you click the More button on the about:addons page.

• https://developer.mozilla.org/docs/Tools/Valence