Wow, thanks for sharing. I just installed Firefox Nightly (v122.0a1) from the Play Store and sideloading does work again. After tapping the Nightly logo several times, an "Install add-on from file" option shows up in the settings. I can even install an unsigned add-on with "xpinstall.signatures.required" set to "false" in about:config. For the longest time, Nightly allowed users to set up the "Custom Add-on collections" workaround to install add-ons from addons.mozilla.org but not sideload add-ons directly, so this is a major improvement.

Edit: Nightly gained the ability to sideload add-ons 2 weeks ago from the pull request at https://github.com/mozilla-mobile/firefox-android/pull/4568. Also, a Mozilla employee has confirmed that sideloading is going to make it to the release channel of Firefox for Android!* Firefox is having an incredible month. It took time, but I'm extremely glad Mozilla is taking user feedback seriously.

* "We do want this feature in Release." https://github.com/mozilla-mobile/firefox-android/pull/4568#...

Oh wow, finally, great. The inability to manually install add-ons was also a major annoyance for add-on development (at least for me), because without that you were restricted to temporary add-on installation via devtools, so you couldn't really seriously test drive your own add-ons for any extended period of time. (Or attempt tracking down some rare bug.)