If you always want Firefox to block images when using mobile data then you can set it in the menu: Settings → Advanced → Show images → choose Only over Wi-Fi or Blocked. This is however tedious if you often quickly want to turn this option on or off.
As far as I know, there‘s no working add-on for Firefox for Android (Fennec) 67.0 at the moment (the mentioned addon Mobile Image Blocker is not compatible anymore since quite some time).
However, there‘s Firefox Lite as an leightweight browser app for Android (seems like it was called Firefox Rocket formerly). Apparently it‘s currently only available in some asian countries through Google Play, but you can also install it via F-Droid from anywhere in the world (either install F-Droid or manually download an install the Firefox Lite APK file from the F-Droid website).
It has an easily accessible option for enabling and disabling blocking images. In the current state it‘s lacking some features like linking it to Firefox Sync for syncing bookmarks for example. Overall it‘s still a good option to reduce traffic when surfing the web on the go.