Stuck "Checking your browser" with Firefox privacy.resistFingerprinting=true

Summary

Clicking Login in Firefox with privacy.resistFingerprinting enabled in about:config shows the "Checking your browser before accessing gitlab.com." page, which refreshes indefinitely.

This started happening recently (in the past week or two). I've re-confirmed the behavior this morning with both Firefox 92.0 (64-bit) on Windows 10 and Firefox Nightly 94.0a1 (2021-09-22) (64-bit) on Linux. It is reproducible with mozregression using mozregression --pref privacy.resistFingerprinting:true -a https://gitlab.com/users/sign_in and reproduces in both the current version and versions from a year ago, which suggests it is not due to a change in Firefox.

Related Issues

gitlab-com/gl-infra/infrastructure#13765
gitlab-org/gitlab#337918 (closed)

privacy.resistFingerprinting was also specifically mentioned by @xianbaum in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13765#note_681307080 3 days ago.

Let me know if there's anything else I can do to help.

Thanks,
Kevin