If you are rooted, you can also force your own add-ons into stable firefox like so:
USER=16201230
COLLECTION=What-I-want-on-Fenix
cd /data/data/org.mozilla.mozilla.firefox/files
curl -o mozilla_components_addon_collection_*.json "https://addons.mozilla.org/api/v4/accounts/account/$USER/collections/$COLLECTION/addons/?page_size=50&sort=-added"
touch -a -m -t 203012300130.00 mozilla_components_addon_collection_*.json
edit: remove fennec fdroid because TIL that it already has the same add-on override that the FF nightly has. So there is no need for this hack if you have fennec.
Tested just now and can confirm that it works. You don't even need root if you are using lineageOS, just enable the root debug shell in android settings.
