It used to be open source, but large parts of it have been relicensed under their proprietary source-available shared source license. The reason why it isn't entirely proprietary is that it's based on Firefox, which is entirely licensed under the MPL. The weak copyleft of the MPL states that all parts lifted from Firefox must remain open source, but the new parts can be proprietary.

Source-available licenses are a type of proprietary license where the code is made public for people to look at, but you're not actually allowed to use it. Users can still contribute upstream, so they're usually parasitic licenses aimed at getting free labour out of the userbase without actually giving back any code to the commons, all while keeping up the illusion of being open source. It sucks.