not sure if this part of stackoverflow for Firefox is for Firefox on Android also.
I've added a long comment to this unfortunately closed bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1387384#c9 explaining why I need the option to place Firefox on Android to "Adoptable Storage".
As written, I can get Firefox installed on "Adoptable Storage" by simply changing Firefox's APK's "common options" - "Install Location" to "Prefer External" with App "APK Editor". No need to recompile anything or similar.
BUT this way with the app "APK Editor" a) "breaks" the upgrade path, I'm getting no updates shown for Firefox anymore in Google's "PlayStore" and b) this workaround with "APK Editor" itself is not able to perform an upgrade, only a new installation, after deinstallation. Dropping all saved credentials, bookmarks etc.
a) is not an issue for me, but, of course, b)
As after ~3 weeks there's not a single answer to my comment on Bugzilla, let me give a try here:
Q1: Where do get get the APK for Firefox on Android, for new versions listed in PlayStore?
Q2: If I change it outside of Android, let's say on Linux, how, if possible, do I get Android to perform an UPGRADE of Firefox, with the newly downloaded and slightly changed APK? Upgrade, preserving user data, not new installation.
Remarks: Installation from "Unkown Sources" is allowed on my smartphone, as I prefer F-Droid Apps whenever possible. Smartphone is NOT rooted and shouldn't be rooted, if avoidable