This is more about responsiveness. It's not just about slow script, if you open a very large text file in a tab, the entire Firefox UI stalls. If you have some

This is more about responsiveness. It's not just about slow script, if you open a very large text file in a tab, the entire Firefox UI stalls. If you have some moderately heavy operation happening in a different tab, scrolling gets choppy across the board, switching tabs is janky, etc etc etc.