Steps to reproduce

1. Open the following mozilla.slack.com comment:
   https://mozilla.slack.com/archives/C01AFKCLWBF/p1603723350120100

2. Click the "Nicole Bjelica" or "Mike Heavers" linkedin.com link.

Expected result

A linkedin.com page should open in a new tab, like in Chrome.

Actual result

Firefox hangs for at least a minute. Sometimes just the Slack tab hangs. Sometimes the browser chrome hangs, too. Firefox will show the yellow busy tab infobar a few times. If you click the "Stop" button, then the new linkedin.com tab will open as expected.

Here is a profile of the hang:
https://share.firefox.dev/2TtgRO0

Looks like the web content process is spending a lot of time in Ion and Layout code.

The hang only seems to happen when clicking the links from the main Slack channel history view. If you click "Replies / View thread" and then click the links from the Slack Thread side bar, Firefox doesn't hang.

This is not a regression because I can reproduce on ESR 68 and 78 and Firefox 82/83/84.

I can reproduce this locally on macOS, but not every time. The second time I click the link it opens immediately. But if I refresh Slack and try again it's back to the slowness. I do see the MutationObserver callbacks in my local profiles as well.

Cameron: Can you reproduce and poke at it a bit?

Emilio, you fixed this issue, right? (Or got the Slack folks to back out some change?)

Yeah, this was bug 1640441 wasn't it?