Facebook malware warning

I clicked on a FB quiz and granted authorization to an app. It logged me off of FB and when I logged back in, I got this message:

"It looks like your computer is being affected by malware. We’ll help you fix the problem to keep your account secure and prevent malware from spreading to friends.
Malware is software that tries to steal personal information and causes problems when you use Facebook. Clicking or sharing links that contain spam can give your computer malware. "

I deleted the app and did some other things to try to fix this, but it still won't let me log in. I didn't use the FB prompted malware programs, only Malware Bytes, which found nothing.

This ONLY happens when using Firefox. I can still access FB in Chrome and on my Android phone. Although some of the things I try to share give me the same virus/malware warning.

A friend signed into his FB account on my computer so it seems to be somehow tied to my sign in...

Any ideas?