The XULRunner Error is usually caused by an incomplete installation/update of Firefox because of bad internet, a sudden system shutdown or an antivirus. The usual solution is to perform a clean install of Firefox. You can customize this article for use with your other laptop - Firefox keeps crashing at startup - Try a clean install https://support.mozilla.org/en-US/kb/firefox-keeps-crashing-startup#w_try-a-clean-install