Not enough jobserver tokens when compiling www/firefox

[bob prohaska]

Experiments  with compiling www/firefox on a Pi3 running
-current have finally stalled with:

gmake[1]: INTERNAL: Exiting with 1 jobserver tokens available; should be 2!

Updating /usr/ports doesn't seem to make a difference, changing
the limit in /etc/make.conf from 2 to 3 changes the message to:

gmake[1]: INTERNAL: Exiting with 1 jobserver tokens available; should be 3!

so /etc/make.conf is being read correctly. The last entries in the log
file are:
gmake[4]: Nothing to be done for 'target-objects'.
gmake[4]: Leaving directory '/usr/ports/www/firefox/work/.build/dom/filesystem'
gmake[4]: Leaving directory '/usr/ports/www/firefox/work/.build/toolkit/library/rust'
gmake[3]: Leaving directory '/usr/ports/www/firefox/work/.build'
gmake[2]: Leaving directory '/usr/ports/www/firefox/work/.build'
gmake[1]: Leaving directory '/usr/ports/www/firefox/work/.build'
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1
 

Ports are at 543527, uname -a reports r362742.

Thanks for reading and any suggestions!

bob prohaska