Found in

• Firefox 105.0a1 (20220801215129) while verifying bug 1779173.

Affected versions

• Firefox 105.0a1 (20220801215129)
• Firefox 104.0b5 (20220801132524)

Affected platforms

• Ubuntu 20.04
• Windows 10x64
• macOS 11

Steps to reproduce

1. Open https://addons.mozilla.com/.
2. Open the debugger.
3. Look for a file in addons.mozilla.com > static-frontend that looks like amo-[numbers].js, and open it
4. Switch tabs inside the devtools between the debugger tab and other tabs multiple times.

Expected result

• There is no slowness or freeze.

Actual result

• Browser starts to slow after switching tabs.

Regression range

• I will search for one ASAP if there is one.

Additional notes

• Attached a screen recording
• Profiler from Windows 10x64: https://share.firefox.dev/3SkfrCv
• Profiler from macOS 11: https://share.firefox.dev/3SdMYyl

Tried searching for a regression range but it's really difficult to find a reliable one. From what I saw Firefox 63.0a1 (2018-07-14) does not slow when switching devtools tabs multiple times. There can be seen some slowness when opening the amo-[numbers].js inside devtools but there is no slowness when switching devtools tabs multiple times.
Last good revision: 04dd259d71db60341016eccf53ced43742319631 (2018-07-14)
First bad revision: 6a320851d377068d46a59ff11d5d5124b219138a (2018-07-15)
Pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=04dd259d71db60341016eccf53ced43742319631&tochange=6a320851d377068d46a59ff11d5d5124b219138a

On older builds like Firefox 55.0a1 (2017-03-08) the slowness is really noticeable when switching devtools tabs.