Because my firefox often freezes for a while when I open something like video section of thedailyshow.com

Given how cheap memory is, and the fact that almost all new devices are multicore I see this as a big positive.

Because "one bad js = frozen browser" is bad

Do you honestly think it is reasonable to expect everyone not to use any plugins? Let me know how they are going to, say, get audio from webapps.

I guess everything is stable, if you just avoid all the features everyone uses.

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.

Firefox has run plugins out of process for years.

It's needed to attract the die hard Chrome users back to Firefox.