@tyil
@Suiseiseki

Freedom to use, modify and redistribute both original and modified software in source code are all four requirements for it to be free software. If you use trademarks to restrict any of these four it is de-facto not fully free software, Thunderbird has components and plugins that are necessary for it to interact with Gmail, Outlook and other major proprietary email providers and if it includes those it can't be free software, if it recommends those then it's doing a disservice to users and it's rather irresponsible. Being mostly free software isn't being free software.