File system paths on Linux are always treated as UTF-8
If you are using Linux, your file system has to be encoded in UTF-8 or Firefox won't understand it and it will generate error messages such as "Error: The profile path is not valid UTF-8. Unable to continue."
If you're not able to configure your filesystem as UTF-8, we recommend using only the letters a-z in the name of the Firefox profile and the download directory.
To change the way your file system is encoded, please consult the documentation for your Linux distribution.