firefox FTBFS during zesty test rebuild

firefox version 50.1.0+build2-0ubuntu1 in zesty failed to build on all arches during a test rebuild of the archive. The build log can be found here:

https://launchpadlibrarian.net/312374945/buildlog_ubuntu-zesty-amd64.firefox_50.1.0+build2-0ubuntu1_BUILDING.txt.gz

The failure seems to be:

DEBUG: configure: error: cannot determine icu version number from uvernum.h header file
ERROR: old-configure failed
*** Fix above errors and then restart with\
               "/usr/bin/make -f client.mk build"
/<<BUILDDIR>>/firefox-50.1.0+build2/client.mk:373: recipe for target 'configure' failed
make[3]: *** [configure] Error 1
make[3]: Leaving directory '/<<BUILDDIR>>/firefox-50.1.0+build2'
/<<BUILDDIR>>/firefox-50.1.0+build2/client.mk:390: recipe for target '/<<BUILDDIR>>/firefox-50.1.0+build2/obj-x86_64-linux-gnu/Makefile' failed
make[2]: *** [/<<BUILDDIR>>/firefox-50.1.0+build2/obj-x86_64-linux-gnu/Makefile] Error 2
make[2]: Leaving directory '/<<BUILDDIR>>/firefox-50.1.0+build2'
client.mk:168: recipe for target 'build' failed
make[1]: *** [build] Error 2
make[1]: Leaving directory '/<<BUILDDIR>>/firefox-50.1.0+build2'
/usr/share/cdbs/1/class/makefile.mk:77: recipe for target 'debian/stamp-makefile-build' failed
make: *** [debian/stamp-makefile-build] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2