There is a privacy setting in firefox that causes this for me on most websites that require photo upload, not all sites, but consistently the same sites.

Ebay for instance, most reverse image searches etc.

in about:config - > privacy.resistFingerprinting

It might not be that setting specifically, but turning that setting to “false” does fix this for me.

There might be a more granular setting that does the same job but i don’t know of it.

Not that i’m recommending turning that off, that’s your call.

I’ve also not tried it on this site specifically.