Just to be clear, this is important because eventually Let's Encrypt wants to no longer have to cross-sign their certificates for them to be considered valid.
For that to happen they have to be added as a trusted CA in most major platforms (and Firefox which has their own CA store for some reason).
> (and Firefox which has their own CA store for some reason).
Firefox has it's own CA store because it's built for all 3 major (desktop) platforms. OSX and Windows have their own but Linux does not and uses Mozilla's.
More importantly, Firefox has its own CA store because it was derived from Netscape, which had its own because they came up with the whole SSL thing way back. The subsystem is still called NSS, even though there's been backbend changes (pkix).
They just use a copy of Mozilla's one on Linux. Of course, distro packages of Chromium if they use the system NSS library may well use some system CA store.
We have also applied to the Apple, Microsoft, Oracle, Blackberry, and Google root programs.
We had hoped to be accepted by all major programs by the end of this year, but even if that happened it would still take years for our root to sufficiently propagate.
For that to happen they have to be added as a trusted CA in most major platforms (and Firefox which has their own CA store for some reason).