>>How much virtual memory Firebird process is used before the crash ?

>>Any messages in the firebird.log near the crash time ?

>

> Vlad, not much memory, about 400k out of 12gb.

  400KB ? Or 400MB ?

400KB is too small to be true ;)

> There’s no other messages in the log before, just the crash msg “terminated abnormally (4294967295)”

> Currently I’m tracing every database being accessed, logging every query trying to find any suspects, but still no luck.

> The crashes still looks to be very random.

> I’m inclined to use a debug version and generate a crash dump as someone suggested.

  There is no "debug version". There is "debug information" - set of .pdb files used by debugger

to analyze crash dump. You don't need it unless you are going to analyze by yourself.

> Would that be helpful? And, are there any tutorials on how to set this up?

  Yes, crash dump usually allows us to know exact reason of crash. 

1. Disable Guardian. It is important ! Make sure it is not running.

2. Collecting User-Mode Dumps:

https://msdn.microsoft.com/en-us/library/windows/desktop/bb787181.aspx

> We have seem a lot of improvement in performance with firebird 3, and I’d really like to avoid having to rollback to 2.5

   Glad you like Firebird 3 :)

Regards,

Vlad