Is that happening when you attempt to install Flashblock from here? https://addons.mozilla.org/en-US/firefox/addon/flashblock/

I was unable to reproduce the issue just now, uninstalling Flashblock required first. The normal way - via the Search results Flashblock - Install The Install button at the bottom of the Details. The suggested link - I didn't see any difference.

That means I see no problem at all now.

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

Adblock Plus Pop-up Addon {web link} Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.

Forum; Adblock Plus Homepage {web link}

It is BACK - the mybrowseraddon - com's firewall-lite webpage appears on SYNCED computer's Firefox after logging into Sync.

Firewall Lite is not installed in the source Firefox - the others are new installs. What is going on? The only obvious remnant of this junk is a bookmark, in case Mozilla asked for the link.

First, shut down sync to prevent spreading.

Clean out the other computers.

mybrowseraddon - com's addons permeat the Firefox List. "Posted by" name differs. icloud-notifier, Facebook Disconnect etc.

Sometimes the Developer website is mybrowseraddon - com or it could be http://add0n - com ---- yes a zero used, or other sites. It is an open source developer site.

If I select Flashblock in the LIST of Addons, FlashBlock should be installed and NOT Firewall Lite.

I have no evidence the addons are malicious or even just lousy.

This GARBAGE is BACK!

I go to SYNC to a new install of Firefox on a Clean Install of Windows 10 and what do I get but that Firewall Lite phishing crap as an open tab, not actually installed.

Includes a form to submit bugs, - EMAIL required.

At least I cannot locate this JUNK in Add-ons Manager.

http://www.clamxav.com/ free malware scanner for Mac OS X https://discussions.apple.com/docs/DOC-3291

You can also try https://www.malwarebytes.org/antimalware/mac/index.html

I've called the big guys to help you. Good luck.

Windows

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.