Steps to reproduce:

Installation or update to Firefox 100 and start the application.

It may be related to our Microsoft Defender for Endpoint Exploit protection rules. However I deactivated it and the issue persists.

Actual results:

It starts but no websites are rendered at all. Also all about-pages or settings menu are not loaded.

Expected results:

Websites or settings menu should be displayed.

Please tell me what logs to share so that you can identify the issue.

about:support and about:third-party would be a start (if you can load them!)

If that's not possible in Firefox 100, maybe the same from Firefox 99 can help.

Another thing you can do is run mozregression: https://mozilla.github.io/mozregression/

It should allow you to pinpoint exactly what change broke your configuration.

(In reply to Gian-Carlo Pascutto [:gcp] from comment #2)

about:support and about:third-party would be a start (if you can load them!)

If that's not possible in Firefox 100, maybe the same from Firefox 99 can help.

Thanks. about-support works and I attached the information below. about-third-party is not working.

Firefox 100.0b1 to 100.0b6 didn't start. The application could not be loaded (0x80000003). Beginning with 100.0b7 it started but showed the strange problems described in my first post.

about-third-party is not working.

It's about:third-party (":" not "-")

Firefox 100.0b1 to 100.0b6 didn't start. The application could not be loaded (0x80000003). Beginning with 100.0b7 it started but showed the strange problems described in my first post.

Thank you, that may be extremely helpful.

(In reply to Gian-Carlo Pascutto [:gcp] from comment #7)

about-third-party is not working.

It's about:third-party (":" not "-")

sorry only a typo, "about:third-party" is not working

Ah yeah, because you can't load any pages. If you have an older, working build, the info from there (from the same page) would work too.

Thinking about the timing of the breakage, 100.0b1 to 100.0b6 would have had Win32k lockdown default-enabled, and then 100.0b7 default-disabled it. The symptoms you see on 100.0b7 mean that we're not managing to start any child/content processes.

It's quite possible that whatever is breaking that is breaking even harder with win32k lockdown enabled, but we need to find out what it is. If you are able to run mozregression which I linked above, it would help to narrow down the change.

FWIW, we have a similar report to this, and those users reported it was indeed caused by Exploit Protection:

"we are running Exploit Protection with custom settings for our organization. If we remove all of those firefox works again, so we will try to isolate which setting is causing the issue."

(In reply to Gian-Carlo Pascutto [:gcp] from comment #11)

FWIW, we have a similar report to this, and those users reported it was indeed caused by Exploit Protection:

"we are running Exploit Protection with custom settings for our organization. If we remove all of those firefox works again, so we will try to isolate which setting is causing the issue."

Thanks I am going to verify this tomorrow.

I can verify that disabling these 3 Exploit Protection Features fixed the problem on our devices:
Simulate execution (SimExec) "Ensures that calls to sensitive functions return to legitimate callers"
Validate API invocation (CallerCheck) "Ensures that sensitive APIs are invoked by legitimate callers"
Validate stack integrity (StackPivot) "Ensures that the stack has not been redirected for sensitive functions"

I think I understand what happened now, and the timing with the Firefox 100 release, please see: https://bugzilla.mozilla.org/show_bug.cgi?id=1770098#c21