The WOFF2 metadata editor is a really cool and simple tool. Starting with a variable TTF font source and converting that to WOFF2 with Google’s WOFF2 tools (https://github.com/google/woff2), the internal font name gets IMO erroneously suffixed with the lightest weight available, be it “Thin”, “ExtraLight” or whatever. Using your tool I was able to clean that up for “Blacker Pro” but there’s a slight problem for another font, namely “Manrope”.
If I download the Manrope family from Google Fonts (https://fonts.google.com/specimen/Manrope) and follow the same steps to convert TTF to WOFF2 and then to remove the “ExtraLight” name suffix using your tool, the resulting WOFF2 file has something in it that makes Firefox go:
downloadable font: fvar: 2 bytes unparsed (font-family: "Manrope" style:normal weight:400..800 stretch:100 src index:0) source: Manrope-Variable.woff2
This warning doesn’t come up with the freshly converted WOFF2 prior to editing the metadata.