Turns out this is harder than it should be.

First of all, you need to run Fennec (from F-Droid) and not Firefox (from Google's Play store), and you need to enable debug mode (which is incidentally a requirement for Firefox sync using a custom server). Note that even if you have previously enabled Firefox sync using a custom server, you must re-enable debug mode again for the custom add-on collection setting to become visible.

To enable debug mode, open Settings, click on About Fennec, then click on the logo a number of times until a popup that says debug mode is enabled pops up.

Now you will find a section called Custom Add-on collection in the Fennec settings.
Create your own custom collection by going to https://addons.mozilla.org/en-US/firefox/collections, login using your Firefox account (you have to create a Firefox "profile", whatever that is).

My add-on collection: ID: 17175532, name: fennec.
Fennec will quit automatically at this point. Restart it, and now you can install any of the add-ons in your collection.

Side-note: it's too bad that Mozilla forces us through these hoops to use extensions on Android. I hope this becomes easier in the future.

• https://blog.dbmiller.org/2021-08-19-using-fennec-or-mull-for-extensible-and-secure-browsing-on-android
• https://reddit.com/r/fossdroid/comments/p7mgec/guide_using_most_all_firefox_addons_on_fennec_on/
• http://hackerpublicradio.org/eps.php?id=3851