Bugzilla

I can't reproduce your issue using Firefox Release v84.0.1 or v84.0.2 or Nightly v86.0a1 (2021-01-09) or Nightly v86.0a1 (2021-01-10) on Windows 10 or Mac OS 11.1 (both ARM and non-ARM processor devices) or Mac OS 10.15. 

We don't have Mac 10.16 (as the performance profile suggests), but I assume that it is actually Mac OS 11:
"As of macOS Big Sur Beta 3, the system version is reported as "11.0" on all Macs running the beta.[citation needed] However, to maintain backwards compatibility, macOS Big Sur also identifies itself as 10.16 to software and in the browser user agent."

This being said, it appears we need more information about your system. 
1. Is it an ARM architecture device? What exact Apple device reproduces this issue?
2. I would like you to test your issue in safe mode and tell us the result.
(https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode)

I don't know if the provided profile confirms the issue on exposes the cause of your hang, but I certainly cannot reproduce it.

I can't reproduce your issue using Firefox Release v84.0.1 or v84.0.2 or Nightly v86.0a1 (2021-01-09) or Nightly v86.0a1 (2021-01-10) on Windows 10 or Mac OS 11.1 (both ARM and non-ARM processor devices) or Mac OS 10.15. I tested it by editing a test bug and saved values (form history, not autofill) on some of the form fields, then edited the bug and clicked on the fields with saved form history values.

We don't have Mac 10.16 (as the performance profile suggests), but I assume that it is actually Mac OS 11:
"As of macOS Big Sur Beta 3, the system version is reported as "11.0" on all Macs running the beta.[citation needed] However, to maintain backwards compatibility, macOS Big Sur also identifies itself as 10.16 to software and in the browser user agent."

This being said, it appears we need more information about your system. 
1. Is it an ARM architecture device? What exact Apple device reproduces this issue?
2. I would like you to test your issue in safe mode and tell us the result.
(https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode)
3. A screen recording might be of some use if you think I've misunderstood your steps.

I don't know if the provided perf profile confirms the issue or exposes the cause of your hang, but I certainly cannot reproduce it.