Upstream support forum thread:
https://forum.palemoon.org/viewtopic.php?f=70&t=25435
Workaround, that will restore ~ 95% of GitHub's lost functionality:
Install github-wc-polyfill-1.0.3.xpi from:
https://github.com/JustOff/github-wc-polyfill/releases/tag/1.0.3
All credit goes to (upstream) Ukrainian developer @JustOff ; however, that extension was created with official Pale Moon in mind, hence its install.rdf file has:
<em:minVersion>28.14.0</em:minVersion>
Should install as is on Mypal 28.14.x (if that's what you want it for ), but to install on latest New Moon 28, one has to edit that line to read:
<em:minVersion>28.10.2a1</em:minVersion>
The add-on itself has been probably inspired by the WebExtension add-on Polly, that was created for Waterfox Classic users to mitigate GitHub's breakage on Oct 14th 2020... Polly targets a Firefox 56 fork, which has a lot more WebExtension APIs than either Serpent 52.9.0 or Centaury; as a result, Polly will install on Serpent/Centaury, but won't work as designed...
Additionally, Waterfox Classic users can (mostly) fix GitHub by toggling dom.webcomponents.enabled to true but, while the same pref does exist in UXP browsers, Web Components support in UXP is extremely unripe/at a very early stage...
FYI, github-wc-polyfill extension embeds a base64encoded copy of
https://unpkg.com/browse/@webcomponents/webcomponentsjs@2.4.4/webcomponents-bundle.js
https://github.com/webcomponents/polyfills/tree/master/packages/webcomponentsjs#using-webcomponents-bundlejs
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Some background:
Before the Microsoft buyout () , GitHub would support the latest version of major browsers like Google Chrome, IE/old Microsoft Edge, Safari, Firefox, and care would be taken to not break the current ESR version of Firefox; additionally, "legacy" browsers like Pale Moon, Basilisk, Waterfox (Fx56-based) were "whitelisted" and treated in a special manner so as to remain compatible, but still on a best-effort basis...
It well appears that now the "old" team of GitHub developers, the ones sympathetic to "legacy" browsers, have been set aside by the new management, which is hardly any surprise as Microsoft have colluded with evil Google, the net effect being that currently GitHub only caters to the four iterations of Chromium that dominate the browser market (Google's Chromium=Chrome, Microsoft's Chromium=[New] MS Edge, Apple's Chromium=Safari and Mozilla's Chromium-wannabe=Firefox [Quantum] ) ...