After installing KDE Plasma on my Ubuntu 20.04, on Gnome the default file manager for Firefox, Chrome and Brave is now Dolphin. Even on a new created user, so the issue might be on /usr/share/applications/
I already changed inode/directory line of the file /usr/share/applications/mimeinfo.cache:
Very strange, it worked only 20 seconds, Chrome launched Files (Nautilus) and then it switched to Dolphin. There might be a KDE process modifying this setting!