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.

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

The increasing prevalence of cookie permission banners is making browsing near intolerable. There is no fool-proof method to avoid seeing them, but here's the methods I've learnt about and my own recommendation.

• "Is there any way to block the cookie agreement popup using Firefox?", Quora question with a few informative answers.
• "Extension that blocks all those 'accept cookie' popups?", Reddit question with good answers.
• "How to stop irritating cookie popups", blog post by Gtarafdar showing how to do it using CSS (I'd consider this the hardcode, manual way).

I stopped using the "I don't care about cookies" (IDCAC) extension by Daniel Kladnik after I encountered a bug on a website that was caused by the cookie consent popup being hidden from view but causing scrolling to not work, and there was no way to report the bug except mailing the developer. The extension having been recently bought out by Avast certainly did not help convince me to not look for alternatives.

• IDCAC does not have any public repo and thus no public issue queue. But the source code is supposedly published under a libre license, but where is the code published?
• https://www.producthunt.com/posts/i-don-t-care-about-cookies-2
• IDCAC has been sold to Avast. This is probably bad news.

Other browser extensions with similar functionality:

• https://consentomatic.au.dk, Firefox add-on. Has public issue tracker on Github (also, 1.2k stars). This is what I switched to from IDCAC. But I was not at all happy with its performance - for most sites it did not hide/remove the cookie popup.
• I Still Don't Care about Cookies is a fork of the IDCAC project, but in contrast to IDCAC it is hosted on Github with a public issue tracker. This will be my choice going forward.
• https://github.com/CodyMcCodington/AutoCookieOptout, Firefox add-on
• https://ninja-cookie.com/, Firefox add-on (appears outdated)
• Cookie Block
• Never-Consent

Tangentially related:

• https://addons.mozilla.org/en-US/firefox/addon/cookie-remover/ (via https://pluralistic.net/2022/06/28/bartlebytron-3000/)
• https://github.com/t-mart/kill-sticky (via ---)