Until now I never had issues with Firefox.

But here in Germany the fanbase of ff ist larger. Maybe that’s why you don’t have problems with most of sites from Europe?!?

Sounds like your nextcloud instance has a low limit for the upload file size. It has to be adjusted in the php settings of your nextcloud server. I set it to 1 GB and have no problem in uploading my videos using auto upload of the nextcloud app.