Hello guys, i'm using Firefox 60.0.2 with Dolphin 18.04. So, when I try to upload any file, Firefox uses the nemo instead of dolphin, even being as a priority in my mime configuration. How should i solve this?

There's usually a separate Firefox - Plasma integration package that you need to install. That or a separate set of steps to follow if such a package isn't provided by your distro.

Which Distro (and version) are you using?

I'm using Archlinux with Kernel 4.17.2

https://wiki.archlinux.org/index.php/Firefox