Firefox 53 fails to build on ARM, and rather than fixing this before shipping it to Fedora 26 (and Rawhide), the Firefox maintainers built it and shipped it with non-x86 arches disabled:
This means Xfce ARM (and probably other ARM images, but Xfce ARM is a release-blocking image) fails to compose, and also that it has no default browser.
There are two possible resolutions to this: fix Firefox to build on ARM, or change Xfce's default browser to something that *is* available on ARM. Obviously we would highly prefer the first solution.
Proposing as a Beta blocker (as an alternative to blocking on https://bugzilla.redhat.com/show_bug.cgi?id=1443938 directly), violates the Alpha criteria "All release-blocking images must boot in their supported configurations" (since a blocking image is missing for now) and "It must be possible to run the default web browser and a terminal application from all release-blocking desktop environments" (just removing Firefox from the package set would not be a sufficient fix).
Discussed in the 2017-05-02 Blocker Review meeting . This is accepted as a blocker in lieu of #1443928 because this more precisely reflects the actual criteria violation. There was a long discussion on it during the meeting  as well as a mailing list discussion  to explain the rationale.
 Sorry for the late update on this.
The actual text of the #AGREED is: "This bug is a conditional violation of the following criterion: "It must be possible to run the default web browser and a terminal application from all release-blocking desktop environments." as well as the "image compose" criterion."
Firefox was patched to support the armv7hl architecture.
This appears to be resolved now; the update has been pushed stable and we have Xfce ARM images again in current composes. I haven't had direct confirmation that Firefox *works* properly, but if there are any problems there, they can be reported separately. Thanks for the build fix!