Elias, in you comment #175 you didn't wrote which version of Firefox you had when you run it for the first time.
Also, have you noticed that if you make a change in
Regional Formats in System Settings > Language > Language Support > Regional Formats
the change takes effect the next time you log in (see my comment #172).
Then check what you get when you run the locale command in the terminal window.
Elias, in you comment #175 you didn't wrote which version of Firefox you had when you run it for the first time.
Also, have you noticed that if you make a change in
Regional Formats in System Settings > Language > Language Support > Regional Formats
the change takes effect the next time you log in (see my comment #172).
Then check what you get when you run the locale command in the terminal window.