Hello nanogram32,

This must be pretty frustrating for you ....

Would you please go to Preferences => Add-ons => Extensions and remove anything that you didn't install yourself.

Then run every malware scan that is listed in this article :

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

(what one doesn't pick up, the other one might)

And please read this article as well :

http://www.2-remove-virus.com/remove-my-inbox-helper-from-chrome-firefox-ie-edge/

'Hope that will help you get rid of that "Helper" (pun intended) ....

The Troubleshoot Firefox issues caused by malware article when viewed for Mac has a "Mac security and malware protection" section that recommends Malwarebytes for Mac to help clean your Mac of potential malware.

I also found a recent Apple Community forum thread, in case it helps: https://discussions.apple.com/thread/250234299 Uninstalling MY INBOX HELPER - Apple Community