My steps was:
- download 45 firefox (https://archive.mozilla.org/pub/firefox/releases/45.0/source/firefox-45.0.source.tar.xz)
- unpack sources
- create project with existing sources:
-- select source root, GNU tool collection
-- forbid pre-build step
-- set "echo clean" and "./mach build" commands for clean&build
-- rest pages is default

IDE start to build project and build command is failed with message:
...
0:04.58 configure:3604: checking whether the C compiler (${IDE_CC} -g3 -gdwarf-2 ) works
 0:04.58 configure:3620: ${IDE_CC} -o conftest -g3 -gdwarf-2   conftest.c  1>&5
 0:04.58 /export/home/tester/test_projects/firefox-45.0/configure: line 3619: ${IDE_CC}: command not found
 0:04.58 configure: failed program was:
 0:04.58 
 0:04.58 #line 3615 "configure"
 0:04.58 #include "confdefs.h"
 0:04.58 
 0:04.58 main(){return(0);}
 0:04.58 configure: error: installation or configuration problem: C compiler cannot create executables.
 0:04.58 *** Fix above errors and then restart with\
 0:04.58                "/usr/bin/gmake -f client.mk build"
 0:04.58 gmake[2]: *** [configure] Error 1
 0:04.58 gmake[1]: *** [/export/home/tester/test_projects/firefox-45.0/obj-x86_64-unknown-linux-gnu/Makefile] Error 2
 0:04.58 gmake: *** [build] Error 2
 0:04.63 0 compiler warnings present.

It seems IDE takes env variables from skipped (pre-build) step.
IDE message log contains message:
Run /bin/sh
	in folder   /export/home/tester/test_projects/firefox-45.0
	arguments    -c sh ./mach build
	host        tester@perf-linux
	environment 
		BASH_FUNC_module()=() {  eval `/usr/bin/modulecmd bash $*`
		CC=${IDE_CC}
		CFLAGS=-g3 -gdwarf-2
		CVS_RSH=ssh
		CXX=${IDE_CXX}
		CXXFLAGS=-g3 -gdwarf-2
		GCONF_LOCAL_LOCKS=1
		G_BROKEN_FILENAMES=1
		HISTSIZE=1000
		HOME=/export/home/tester
		HOSTNAME=perf-linux
		INPUTRC=/etc/inputrc
		KDEDIRS=/usr
		KDE_IS_PRELINKED=1
		LANG=en_US.UTF-8
		LD_LIBRARY_PATH=/var/tmp/dlight_tester/730a3254/Linux-x86:/var/tmp/dlight_tester/730a3254/Linux-x86_64
		LD_PRELOAD=libBuildTrace.so
		LESSOPEN=||/usr/bin/lesspipe.sh %s
		LOADEDMODULES=
		LOGNAME=tester
		MAIL=/var/spool/mail/tester
		MODULEPATH=/usr/share/Modules/modulefiles:/etc/modulefiles
		MODULESHOME=/usr/share/Modules
		PATH=/usr/local/bin:/usr/lib64/qt-3.3/bin:/usr/kerberos/sbin:/usr/kerberos/bin:/bin:/usr/bin:/usr/dev_infra/platform/bin:/usr/dev_infra/generic/bin:/usr/local/bin:/usr/X11R6/bin:/usr/local/ade/bin
		PWD=/export/home/tester
		QTDIR=/usr/lib64/qt-3.3
		QTINC=/usr/lib64/qt-3.3/include
		QTLIB=/usr/lib64/qt-3.3/lib
		SHELL=/bin/bash
		SHLVL=1
		SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
		SSH_CLIENT=10.162.82.101 63135 22
		SSH_CONNECTION=10.162.82.101 63135 10.162.100.219 22
		TZ=Europe/Moscow
		USER=tester
		_=/usr/bin/env
		__CND_BUILD_LOG__=/var/tmp/dlight_tester/730a3254/exec2910569188785064530.log
		__CND_TOOLS__=gcc:c++:g++:clang:clang++:icc:icpc:ifort:gfortran:g77:g90:g95:cc:CC:ffortran:f77:f90:f95:ar:ld