I wasn't able to reproduce this behaviour in the latest version of Firefox, everything is working fine. Please, try clear your browsers cache and also try to restart the application from Site Manager.
However, there were really some bugs, when the culture was not switched correctly, but I am not sure if they fix your issue as well:
Content - Document data was in special cases incorrectly saved to the wrong language version if a user switched the culture in another browser window. (fixed in hotfix 24)
Content - There was an insufficient check for preferred culture code in special cases. (fixed in hotfix 21)
Could you please try to apply the latest hotfix for 55 version?