I’m having trouble to find a bluetooth dongle at least 3.0 that needs no propietary firmware. It’s easy to find dongles advertised as linux compatible or users that claim that an specific brand works fine in linux, but the problem is that many of them are using propietary firmware without their users being aware because their distributions have already installed propietary drivers or firmwares, or ask users to install them and they just do it. I use debian main repository (without non-free software) in which I failed to make work a couple of linux compatible advertised dongles because debian ask me to install a propietary firmware. So if anyone knows for certain that some brand that needs no such a software in linux I’ll apreciate your help.
Just to clarify. In-kernel drivers is not the same as open source firmware. Most bluetooth dongles use the in-kernel driver, but require proprietary firmware to be loaded before they work. Most of that firmware is present in the linux-firmware packages/repository, but the setup would no longer be FOSS only.