Stuck "Checking your browser" with Firefox privacy.resistFingerprinting=true
### Summary
Clicking [Login](https://gitlab.com/users/sign_in) in Firefox with [`privacy.resistFingerprinting` enabled](https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting) 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](https://mozilla.github.io/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
`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
GitLab AI Context
Project: gitlab-com/gl-infra/production-engineering
Instance: https://gitlab.com
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