Seems fine to me.
I think this will be a great add-on to Firefox.

Не плохо работает! Блокирует переход на сайты содержащие подозрительные файлы для загрузки!

è un filtro molto efficace e nello stesso tempo maneggevole

funciona bien, lo recomiendo.

This extension is legitimate

https://forums.malwarebytes.com/topic/218383-malwarebytes-extension-for-firefox/

I've always appreciated the fine work that Malwarebytes does.

My first impression was very bad, regarding the add-on size (+5MiB).

However, my second impression changed when I tested both, performance and detection. Compared to other similar add-ons (Bitdefender, Norton, Avast, Qihoo etc), Malwarebytes in my tests performed better.
Don't expect 100% detection or zero impact performance. All security layer is imperfect. But so far, Malwarebytes seems to me the best add-on in its kind.

The "advertising/tracker protection" was very interesting, definitely a competitive differential with similar add-ons.
And compared to other dedicated tracking blockers, in blocked quantity Malwarebytes performed equal or even better (the only problem is quality, there is not info about what is blocked).
Most important, Malwarebytes is the first tracking blocker not showing redundant info with other add-ons. For example, I use UMatrix, and when UMatrix is "on" or "off", the number of trackers blocked by Malwarebytes change. Very nice indeed.

Please, I would like to see in next versions, details about what the add-on detects (malwares) or blocks (trackers).

Also, I would like to see a better description of the add-on, with more info about how it works (cloud scanning?, behavior scanning?, real-time scanning?, etc?). The same for the tracking blocker, I would like info about what is and what is not blocked. And more info about privacy and what is collected by the add-on, also will be more than welcome.

Thank you!

PS: Lot of people in the web saying it could be a fake add-on.
Better to uninstall it immediately!... and to wait for an official announcement from Malwarebytes.