In a statement, FSF executive director John Sullivan said the company had made renewed security promises in the past as well.
He was referring to a Microsoft announcement last week that it would be expanding encryption for its products in the wake of the revelations that America's National Security Agency has been conducting blanket surveillance of many countries. Microsoft and several other big technology outfits have also been exposed as co-operating with the NSA.
Sullivan pointed out that proprietary software like Windows was fundamentally insecure, not because of Microsoft's privacy policies but because the code was hidden from users whose interests it was supposed to secure.
"When no-one except Microsoft can see the operating system code underneath, or fix it when problems are discovered, it is impossible to have a true chain of trust," he said.
"If the NSA revelations have taught us anything, it is that journalists, governments, schools, advocacy organisations, companies, and individuals, must be using operating systems whose code can be reviewed and modified without Microsoft or any other third party's blessing. When we don't have that, back-doors and privacy violations are inevitable.
"While the Microsoft announcement does promise 'transparency' to reassure people that there are no back-doors in Windows, this is no solution. Transparency in the Windows world normally means self-reports commissioned by Microsoft, or access granted to outsiders covering very limited portions of source code under strict agreements that limit sharing that information."
Sullivan emphasised that freedom and security could not be guaranteed by just being allowed a peek at the code. "Microsoft has demonstrated time and time again that its definition of a 'back-door' will not be the same as yours. Noticing that the back-door is wide open will do you no good if you are forbidden from shutting it," he said.
"The solution after Microsoft's announcement is the same as it was before its announcement. Just like Microsoft's former chief privacy adviser, switch to a free software operating system like GNU/Linux, and don't look back," he said.