At any rate, Mozilla seems to be very intent to kill Firefox if you look at the latest Play Store reviews so I won't be putting too much effort into it. Once the dust settles and there is a clear path to make the extension work in the long run - maybe.
So full add on support for Android Firefox is in the pipeline. However, you can use any add on you like right now on Android with a bit of a workaround.
First of all, you need to download Firefox Nightly, which is essentially the alpha developer version.
Direct link:
https://www.mozilla.org/en-GB/firefox/channel/desktop/Google Play link:
https://play.google.com/store/apps/details?id=org.mozilla.fenixYou then need to go to
https://addons.mozilla.org/en-US/firefox/collections/, create an account, and then create a new collection with a unique name. Search for all the add ons you want to use, and add them to your collection, BPIP included. Make a note of the unique name you gave the collection, and the 8 digit number in the URL.
Open your Firefox Nightly app, go to settings, scroll to the bottom and select "About Firefox Nightly", and tap the Firefox logo five times until you get a pop up saying "Debug menu enabled".
Go back to settings, scroll down to "Advanced" and select "Custom Add-on Collection".
Enter the 8 digit number from above in the "Collection owner (User ID)" field, and the name you gave your collection in the "Collection Name" field, and hit OK.
The app will apply the changes and then shut down.
Relaunch the app, go to add ons, and you will see all the add ons from your collection listed. Tap the plus sign next to each one to install.