Came across WebGL issue in PureOS 2.1 with my new Librem 15 (possibly linked to recovery mode use). In case anybody else comes across WebGL Not Available errors, e.g. here https://cesiumjs.org/NewYork/index.html, then heading to about:config in Firefox, resetting the gfx.crash-guard.status.glcontext to 0 and restarting the browser fixed it for me. Credit: https://bbs.archlinux.org/viewtopic.php?id=207637