Free Software: Free (as in freedom, not price) software puts the user in control by allowing you to use the software for any purpose, modify it, and redistribute it. This also implies that you can examine the source code and see what it does and how it works. This represents a baseline requirement for recommendation in this guide.
Supports Interoperability: Interoperability increases freedom and usefulness by allowing different systems to work together. When a service is hostile to other services interoperating, particularly a communication service, this places users in a walled garden, which limits choice and can make it difficult to switch to or from an alternative.
Recommends Proprietary Software: When a program recommends the installation of proprietary programs or add-ons without clearly warning the user, it can result in the user accidentally installing such software without understanding the implications.
In this case, the desktop version of Thunderbird recommends proprietary add-ons through Mozilla’s extensions directory.