UXP browsers like NM28/St52 are no longer supported by Microsoft-owned GitHub, as they now only target the four "major" browsers, all some form of Chromium forks (Google Chrome, Mozilla Firefox[Quantum] > 68.0, Opera, Microsoft [Chr]Edge) ; they're now using Chromium-only frameworks like WebComponents/Custom Elements/Shadow Dom etc, that the UXP platform doesn't support currently, and is, to be realistic, still far away from supporting in the (near?) future...
In the specific case of GitHub, a true life-saver is the legacy extension referenced just three posts above by @Sampei.Nihira, github-wc-polyfill, currently at version 1.1.7 ; unlike NM28, in St52 it would install (and eventually update) right out of the box, without tinkering with its install.rdf file; you have to be, though, on a fairly recent version of Serpent 52, as it relies on APIs found in relatively recent UXP snapshots (anything within the last 4 months should be OK, if you ask me...).