Edit → Edit current font opens the selected font — or the one highlighted in the list — in the font editor named in Settings > Configure Fontmatrix > Tools.

Where that editor cannot be reached — inside a Flatpak, where no program of the system is the application's to start, or simply when no editor is configured — the font file is handed to the desktop instead, which asks which application should open it and gives that application the right to write the file, so that an editor can save its changes.