From this link:

I have some sort of bug what underlines/links a random word in a page and then gives an add for Apple stuff,how do I get rid of this? | Firefox Support Forum | Mozilla Support

I found this suggestion:

How To Remove Text Enhance Malware - Text Enhance Virus Removal Instructions | Malware Removal - Software & Tutorials

Make sure there are no add-ons in Firefox that you didn't put there or aren't sure what they do.

Tools > Add-ons > Extensions

Disable anything you don't recognize. Also double-check:

Toos > Add-ons > Plugins

similarly.