Firefox not happy with El Capitan.

It may be an incompatible FF extension.. You would have to check them one by one.

If you are facing the "stuck" Firefox that will not force quit or allow you to restart, here are two steps I had read somewhere to resolve this:

In terminal:

enter the following commands and your password:

sudo killall launchservicesd

and then

sudo killall Dock

This should reset the Dock (keeping everything in place, no worries).

I have trie this and it works.

everytime i print it crashes. tried starting firefox in safe mode with all add-ons disabled - still crashed. tried doing the disk utility first aid to verify and repair permissions - still crashing. Tried to update firefox - no update available. Safari seems to work. Not sure. I haven't tried resetting the NVRAM which i'll try and update.

FWIW, the current version of Firefox is 48.0.2.

Sorry if this is late, it looks like this post is just over a year old, but I just started having a problem. I thought I was alone here, but then noticed that taylor-design wrote about exactly the same thing:

taylor-design wrote:

In the "bug fix" update 10.11.1 a FireFox crash goes much deeper. If I try to relaunch FireFox the icon will just bounce in the dock. Activity Monitor will not show the process but Force Quit will. Force Quit will not, however, force quit it no matter what. At this point I cannot restart or shutdown, and have to do a hard reset via the power button.

It seems like this issues is over a year old, since I am totally up-to-date and still experiencing it.