OS freeze/deadlock when hitting Alt+Tab right when Mozilla Firefox 3.6.28 started up

0.4.13-RC-40-g63f46f3 gcc dbg lin

Reproduction steps

• have 1 arbitrary app running in background, e.g: explorer filebrowser
• Start Firefox main app and right when it shows its GUI try to switch foreground app via hitting Alt+Tab multiple times

I just tried to reproduce CORE-14129 with "Mozilla Firefox 3.6.28" from rapps.
(In my earlier tests I could only ever repro CORE-14129 with FF28 or FF48.0.2, but up to now I never saw it with the older Firefox 3.6.28.

When I tried it today with FF3.6.28 + 0.4.13-RC-40-g63f46f3 gcc dbg lin, I got no BSOD 0x1E like in CORE-14129, but instead:

• the system totally froze, I could not even move the mouse pointer anymore, but the OS did not break into debugger yet, it was like some kind of endless loop or mutex deadlock situation
• I waited a bit, and then broke into debugger intentionally and took backtrace, then continued, and repeated that multiple times, the resolved callstack looks always the same
  0.4.13-RC-40-g63f46f3_deadlock.log
  0.4.13-RC-40-g63f46f3_deadlock.png

I decided to create a fresh ticket instead of adding more noise to the other one with a different callstack.
If you are sure, that both are the same issue, then feel free to resolve as duplicate.
Otherwise I hope, that you can learn a bit more from this slightly different log here.