hello, it's probably important to add that this malware doesn't spread by using a vulnerability in chrome or firefox but by social engineering & by tricking users into installing malicious software under a false pretext. this by itself isn't anything unusual and happens all over the web all the time - the vector of infections seems to be quite effective though, since you'll be more likely to install something that comes from one of your social contacts than from a random website (which might claim a download is necessary because a plugin is missing or by impostoring a false update notice).

if it really is only hooking into firefox as an addon it should be fairly easy to remove - just press the shift key while firefox is launching in order to enter safe mode, go to firefox > addons > extensions and remove anything there that looks suspicious & you haven't added manually: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

if your system has been compromised it is also important to change all your stored passwords...

Troubleshoot Firefox issues caused by malware

edit: the addon in question got already blocked by mozilla: https://addons.mozilla.org/firefox/blocked/i444

Thanks Philipp,

The information in your edit about the blocked addon was what I expected to find before posting the question. I searched the term "Facebook malware" and checked a couple of the articles that were suggested before posting the question.

May I ask how you came to locate it so I can find such things in the future?

i found it by looking into the block request of latest entry of blocklisted addons - this list is generally available under https://addons.mozilla.org/firefox/blocked