Hi CardealRusso. Thanks for the tip. It made me realize that Juanito posted his mozconfig (available 
here) and that it has some options (e.g., 
--disable-gconf) not included in the BLFS version of mozconfig.
If I use Juanito's build notes, his mozconfig, and follow the steps exactly, everything works:
$ CC=clang CXX=clang++ ./mach configure
---snip---
 0:30.00 FasterMake backend executed in 0.61s
 0:30.00   9 total backend files; 9 created; 0 updated; 0 unchanged; 0 deleted
 0:30.00 Total wall time: 10.67s; CPU time: 10.66s; Efficiency: 100%; Untracked: 0.36s
Configure complete!
Be sure to run |mach build| to pick up any changes
Hi cute curtis, I bet if you use Juanito's mozconfig and follow the steps carefully, you will succeed. Also, note this starred comment at the top of Juanito's build notes:
*** needs utf-8 locale ***
If you are not using a utf-8 locale, please try using one. Run 
locale in a terminal to see your current locale. If yours is not a utf-8 locale, use the  getlocale.tcz  extension to create an extension (e.g., mylocale.tcz) for your desired locale. Then put 
mylocale.tcz  in 
tce/optional/, add 
mylocale.tcz to your 
tce/onboot.lst file and reboot. When rebooting, include a boot code that specifies your desired locale (e.g., 
lang=en_US.UTF-8).