Renaming flags was a nice new feature introduced in 2.1.45. I don’t know if this is was intentional or not, but actually the flags names are shared across profiles: if you rename, say, the red flag as “Political content” and swicht your user profile, the new profile will also show the red flag as “Political content” in the brower.
Since user profiles are often used by different people using the same computer, I think that being able to save your custom flag names as part of your profile data would make more sense.