Firefox testing - offscreen surfaces and OMTC

[drago01]

On Mon, Aug 24, 2015 at 8:54 AM, Martin Stransky <stransky at redhat.com> wrote:
> On 08/21/2015 08:28 PM, Thomas Daede wrote:
>>
>> I've been running nightly with this enabled for quite a while on Intel
>> and it's been fine.
>>
>> Note that OMTC is required for e10s.
>
>
> If you mean OMTC by "this" then you're right - it works fine on nightly
> because nightly is built with in-tree cairo. Fedora it built with system
> cairo which causes crashes with OMTC enabled. First build which supports
> in-tree cairo & Gtk3 is FF41.

Why? What's the difference between the in-tree and the system cairo?
Ist it newer? Older? Patched? Or just built with a different
configuration?