Warning! In this video I demonstrate a conversion process using a Sketchfab Pro account. The Sketchfab process I show only works with a paid account. The free Sketchfab account will not convert your file to gltf.
As an alternative, use a free Google Poly account. It will serve the same purpose as Sketchfab, but a free account will suffice. Note that the free Google Poly only accepts 100mb files, a reasonable file size considering that Mozilla Hubs only supports 126mb scenes.