www/firefox 53 build is borked

[Paul Goyette <paul%whooppee.com@localhost>]

It gets as far as

	...
	js/src> creating ./config.data
	js/src> Creating config.status

	Creating config.status
	Reticulating splines...

and then it just sits there with 5 python-27 processes each sucking up 
100% of a CPU core/thread (I have MAKE_JOBS set to 8).

I waited more than 90 minutes with no apparent progress, and then just 
killed the build.



+------------------+--------------------------+----------------------------+
| Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:          |
| (Retired)        | FA29 0E3B 35AF E8AE 6651 | paul at whooppee dot com   |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd dot org |
+------------------+--------------------------+----------------------------+