Shame that the extension doesn't seem to be compatible with Firefox on Android, since Reddit's mobile site is borderline unusable for displaying images, doubly so if you want to zoom/scroll them.
This suggests it only works in Nightly, but it also works in Fennec F-Droid, so frankly you should consider switching to that if you can. (I wish Fennec F-Droid could just allow arbitrary extensions and extension side-loading, but I guess it would be non-trivial. Thanks Mozilla!)
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...
This suggests it only works in Nightly, but it also works in Fennec F-Droid, so frankly you should consider switching to that if you can. (I wish Fennec F-Droid could just allow arbitrary extensions and extension side-loading, but I guess it would be non-trivial. Thanks Mozilla!)