[Firefox] Breaks Tampermonkey on GitHub, possibly other sites

[rmenessec]

When GitHub (*.github.com) has any restrictions applied, whether via a rule for github.com or via restrictions on "Default," Tampermonkey's overlay badge (script count) does not appear, and userscripts are not applied. So far, I've ruled out bad interactions with Firefox containers and with First-Party Isolation.

I don't know which, if any, other sites are affected. Google search is not affected, and userscripts continue to run on www.google.com. I don't have time to test right now to see whether Greasemonkey is affected by this issue.

I'm using Firefox Developer Edition 60.0b13 amd64 on Ubuntu Linux 16.04 LTS amd64 at time of writing. To provide as much additional information as possible, I've uploaded the contents of about:support as a public gist, here, in the original XHTML format for ease of viewing. If you prefer the contents as plaintext or as "raw" JSON, I would be happy to provide that as well.

If there is other information I can provide, please let me know. Thanks in advance for your help.