2.5. Floorp is not completely open-source same. Floorp's a part of codes are protected by copyright law and is not licensed under an open-source license. You may not use part of Floorp's code in your own projects without permission from the Licensor.

2.6. If MPL2.0 or any other open-source license notation is present in the file, the file is available as open source.

It seems like their modifications and the resulting application are proprietary.

The MPL is an open-source license designed to protect its own code. If you modify MPL code, you need to publish those modifications under the MPL. That's the focus of the license. There's no "viral" requirement to affect the licensing provisions of the rest of your code.

It seems this is a legal way of using MPL code.