WebGL extensions are not disabled anymore with Tor Browser based on >= ESR 78

It seems the webgl.disable-extensions which we still set and which was supposed to disable WebGL extensions is gone. The option to disable those extensions got removed it seems during a refactoring.

Thus, if one checks e.g. on https://webglreport.com/ extensions are enabled now. We should

i. think about what we want to do now (should we write a patch to disable them again?). ii. go over our other WebGL prefs we set and check whether they are still used.