Where are you going for that to popup?

It comes up on the left difr of the browser...it has a Facebook button, twitter, and email with a plus sign to add more. When going to the plus sign in the bottom corner there is an add this button that when hovered over with the mouse shows addthis.com.

Please check in Programs & Features (Search) and see if you have that listed. If so uninstall it.

Please use more than 1 scanner as each uses diff tech :

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

Save your Report and google each before deleting anything as do not want to delete something you need, If need help :

• https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/

Post in only 1 forum, then wait.

Please let us know if this solved your issue or if need further assistance.