User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:76.0) Gecko/20100101 Firefox/76.0
Steps to reproduce:
I generally start several copies of Firefox using different profiles when my computer boots up in the morning. So I turned on my computer.
This morning, there was a firefox update available. Two of the profiles were successful in starting the old version, no doubt they both started updating, and must have gotten far enough fast enough to cause problems for all the others. When I came back to look at the computer after turning it on, I found those two, running, but About Help indicated I should restart to update. The others had all crashed with a dialog stating:
The procedure entry point ??0MutexImpl@detail@mozilla@@QEAA@XZ could not be located in the dynamic link library C:\Program Files\Firefox\xul.dll.
I closed all those dialogs (which produced another dialog "Couldn't load XPCOM", closed it too), and then there was no sign of those instances of firefox.
I clicked restart on the two instances that were running, and ready to update, and they each then crashed in the same manner.
After all firefox instances were crashed and gone, as above, I started one of them manually (instead of the batch file that usually does it), and it crashed in the same manner. I tried several times with no change in behavior.
I have DevEd and Nightly both installed, they each started fine, noticed the update, I clicked restart, and they restarted fine with the new version. But this one one copy of each, done sequentially.
It would seem that the interlock between different instances of firefox updating concurrently has broken, or else the new release today is simply broken.
They should have all started up, with the old or new version of firefox without crashing. Generally in the past when updates occurred, I would find one copy ready to restart for the update, sometimes one or more copies would report that firefox was being updated by another instance, and sometimes one ore more copies would be running the new version (not often).
I marked this bug as 75 branch, because I'm writing the bug report with 76 branch (DevEd).