build_firefox: update .mozconfig with number of cpu
build_firefox: update .mozconfig with number of cpu

gunesacar/tor-browser-bundle-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,gunesacar/tor-browser-bundle-testsuite,gunesacar/tor-browser-bundle-testsuite,gunesacar/tor-browser-bundle-testsuite