Both of these variables have definitions like "$(DEPTH)/toolkit/xre/$(LIB_PREFIX)xulapp_s.$(LIB_SUFFIX)" (in the Makefile world) or "%s%s.%s" % (CONFIG['LIB_PREFIX'], 'xpcomglue_s', CONFIG['LIB_SUFFIX']), (moz.build world). I believe all assignments always have LIB_PREFIX/LIB_SUFFIX attached to all values, so we mind as well pull that out of the leaf files and put it into the mozbuild classes for those variables. These would result in: SHARED_LIBRARY_LIBS = [ '../parentalcontrols/parentalcontrols_s', # objdir-relative path example '/accessible/src/xul/accessibility_xul_s', # DEPTH-relative path example ] SDK_LIBRARY = 'xpcomglue_s' Note some directories assign values to these variables from configure (things like MOZ_ICU_LIBS), so those would need to be updated as well.