User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:63.0) Gecko/20100101 Firefox/63.0 Steps to reproduce: 100% reproducable on my machine - simply switch user accounts on MacOS 10.14.1 while Firefox is active on screen (in a space) and when returning to that user account, Firefox is no longer responsive. Actual results: Browser non-responsive; force quit necessary. Expected results: The browser keeps working.

I tried a guest user account, but that would only let me use Safari. I assume I have to create a regular new user account in order to see the issue?

Hi, the bug is apparenent to me when using two primary user accounts, both with admin privileges.

This is the same as this bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1403495

Nevermind I thought it was a different bug. I am having the exact same behavior. It happens with admin accounts as well as regular user accounts. It happens almost every time I switch users. IT has been doing this for some time but my current configuration is: Your user agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:63.0) Gecko/20100101 Firefox/63.0

...also note to see the behavior most consistently have the browsers in each session loading a page with lots of JavaScript like Facebook, YouTube, etc.

Hi HankG, I reproduced the issue using one regular user and one guest account. Opened for both users Firefox Release 63.0.3. using different and new clean profiles (default for guest user and test profile for the other). Note that it may take several retries to switch back and forth between profiles but in the end, Firefox becomes unresponsive for one of them for like 20 seconds or more... As the reporter said, makes sure to have Facebook and Youtube tabs opened. Worth mentioning that I could not reproduce it on latest Nightly 65.0a1 (2018-12-03) (64-bit). HankG, can you please also try to reproduce it on Nightly? Here is a link to the latest version: https://www.mozilla.org/en-US/firefox/65.0a1/releasenotes/ The issue can be reproduced on Firefox Release 63.0.3 and latest Beta 64.0b14 (64-bit) on Mac OS 10.14. Nightly doesn't seem to be affected nor Mac OS 10.13.

Timea, I have been running the nightly build 65.0a1 (2018-12-03) (64-bit) for the past several hours trying to reproduce the bug and I have not been able to. I will continue using it for the next few days and see if it doesn't come up. On my machine it was happening so regularly (I constantly switch back between accounts throughout the day) that I was going to have to start using Safari for a little bit. So the fact I haven't been able to reproduce it on the nightly build over the last several hours is a good sign.

This appears to be a duplicate of bug 1422855. It would be great to get a regression range when this started occurring, but also when this started working again if this is indeed fixed in current Nightly. See bug 1422855 comment 23 and bug 1422855 comment 25.