If you look at the MDN docs options.serverCertificateHashes is not yet supported in Firefox (although IIUC it’s in beta! 1873263 - Webtransport: serverCertificateHashes does not work as expected). This is required for talking to a kubo node since without it you’d need to have a CA signed certificate.
So if this indeed made it into Firefox 125 you should only have to wait a few weeks until it’s released Firefox 125 for developers - Mozilla | MDN. Otherwise you should be able to download the Firefox beta and try it out.
Initial testing shows that WebTransport should work much better in Firefox than Chromium at the moment due to an ongoing issue with Chromium in need of resolution Chromium (feel free to star the issue so they know people care).