To edit UTF-8 files you need to configure an external editor such as notepad or Edit++
The internal editor in FlashFXP v4.x does not support UTF-8, it is ANSI only.
We're working on releasing a Unicode compliant edition of FlashFXP with full UTF-8 support but it is this very same problem that is causing us major delays/headaches with the release.
FlashFXP v5.0 beta can be downloaded from our website at
https://oss.azurewebsites.net/download-beta