Thread:User talk:Minh Nguyen/MediaWiki:Accountinfo-user-agent/reply

They're both correct, but "Mozilla Firefox trên Windows" means "Mozilla Firefox on Windows" while "Mozilla Firefox cho Windows" means "Mozilla Firefox for Windows". Yes, you're using the Windows build of Firefox, but more importantly you're using Firefox while using Windows.

If those Google results are to be relied on, there are 123 uses of "Firefox trên Windows", 75 of "Firefox cho Windows", and 54 of "Firefox trong Windows", not counting duplicates. (Make sure to go to the last page of results to see the real total.)