$ ./mach build-backend -b CompileDB
 0:00.07 /home/dannas/code/firefox/obj-x86_64-pc-linux-gnu/_virtualenv/bin/python /home/dannas/code/firefox/obj-x86_64-pc-linux-gnu/config.status --backend CompileDB
Traceback (most recent call last):
  File "/home/dannas/code/firefox/obj-x86_64-pc-linux-gnu/config.status", line 1256, in <module>
    from mozbuild.config_status import config_status
  File "/home/dannas/code/firefox/build/mach_bootstrap.py", line 370, in __call__
    module = self._original_import(name, globals, locals, fromlist, level)
  File "/home/dannas/code/firefox/python/mozbuild/mozbuild/config_status.py", line 21, in <module>
    from mozbuild.base import MachCommandConditions
  File "/home/dannas/code/firefox/build/mach_bootstrap.py", line 370, in __call__
    module = self._original_import(name, globals, locals, fromlist, level)
  File "/home/dannas/code/firefox/python/mozbuild/mozbuild/base.py", line 18, in <module>
    from mozversioncontrol import get_repository_object
  File "/home/dannas/code/firefox/build/mach_bootstrap.py", line 370, in __call__
    module = self._original_import(name, globals, locals, fromlist, level)
ImportError: No module named mozversioncontrol

$ locate mozversioncontrol
/home/dannas/code/firefox/.hg/store/data/build/docs/python/mozversioncontrol.rst.i
/home/dannas/code/firefox/.hg/store/data/python/mozversioncontrol
/home/dannas/code/firefox/.hg/store/data/python/mozversioncontrol/mozversioncontrol
/home/dannas/code/firefox/.hg/store/data/python/mozversioncontrol/mozversioncontrol/____init____.py.i
/home/dannas/code/firefox/.hg/store/data/python/mozversioncontrol/mozversioncontrol/repoupdate.py.i
/home/dannas/code/firefox/js/src/build_DBG.OBJ/_virtualenv/lib/python2.7/site-packages/mozversioncontrol.pth
/home/dannas/code/firefox/js/src/build_OPT.OBJ/_virtualenv/lib/python2.7/site-packages/mozversioncontrol.pth
/home/dannas/code/firefox/js/src/build_arm_DBG.OBJ/_virtualenv/lib/python2.7/site-packages/mozversioncontrol.pth
/home/dannas/code/firefox/js/src/x86-32_build_DBG.OBJ/_virtualenv/lib/python2.7/site-packages/mozversioncontrol.pth
/home/dannas/code/firefox/obj-spider/_virtualenv/lib/python2.7/site-packages/mozversioncontrol.pth
/home/dannas/code/firefox/python/mozversioncontrol
/home/dannas/code/firefox/python/mozversioncontrol/mozversioncontrol
/home/dannas/code/firefox/python/mozversioncontrol/mozversioncontrol/__init__.py
/home/dannas/code/firefox/python/mozversioncontrol/mozversioncontrol/__init__.pyc
/home/dannas/code/firefox/python/mozversioncontrol/mozversioncontrol/repoupdate.py
/home/dannas/code/firefox/python/mozversioncontrol/mozversioncontrol/repoupdate.pyc