I personally found that the new “Selected Photos Access” feature of Android 14, which allows you to give apps permissions only to the photos you intend to upload, is buggy on some apps. So far it seems to be broken on the Tumbr and Firefox apps.
If you also have that issue, at least with Firefox the Nightly version (which you can get from the Play Store) solves that.
I would recommend Mull first, and Fennec second, if you care about privacy.
Use Mull, have Fennec as backup if a site doesnt work