Form MSDN : SPEventReceiverBase.DisableEventFiring Method
Prevents events from being raised.
So someone could think that all events are disabled (all items, all events, all receivers).
But: How DisableEventFiring / EventFiringEnabled works
The setting (DisableEventFiring in 2007 and EventFiringEnabled in
2010) must be for the current event handler; if it was the entire list
there’d be all sorts of timing issues and weirdness, and a thousand
developers would already have screamed in agony.
This is interesting, as it leads to an interesting scenario – if you
were creating additional threads from your Event Receiver’s thread,
then event firing will not be disabled on those. I can’t think why
you’d want to do that, but there you go.
And one more to verify: Disable event firing in SharePoint when updating a list item outside of an event handler
So i whipped out Reflector and started investigating. Upon
disassembling Microsoft.Sharepoint.dll, I discovered that the above
mentioned method actually sets a static, thread-specific, property
of SPEventManager class called EventFiringDisabled. SPEventManager
class is responsible for instantiating event handlers and delivering
events to them.
So second thread created from Event receiver will ignore this property.
I even explored sharepoint.dll (126.96.36.199) and I don't see any changes there except obsolete methods.
This all makes sense to me and I can verify this behaviour but only from my memory of past experience because I don't remember screaming about it :) . However I strongly recommend that you preform some tests on your environment just to be sure.