I was so looking forward to fix up ED-E and have him as a companion. Now I remember why I have my perception to 1.I hate having the red blips on the screen when they are so far away. So my question is, where does ED-E go when you let him go or fire him? I may want him later, but right now, I feel like beging the lone wanderer right now.

Also if a companion gets wounded, how do they heal? Do they heal over time, or do I have to give them stim packs?

ED-E goes back to Primm (at least he did for me). Rex went to the Lucky 38. They both heal over time without requiring stimpacks.

You choose for a companion to either go their original home or in the case of ED-E, Primm. If you've gained a home at the Lucky 38 then you can choose for any/all of your companions to go there after they have been dismissed. Since the UI for ED-E is a tad confusing you have to tell it to go to your "Home Base" for it to go to the Lucky 38 that is either the first or second option, the other one of the two has ED-E go to Primm.

One thing I like immensely about NV over F3 is your companions never really die, they just get knocked unconscious until the battle is done. They heal quickly over time when not in combat but will have to rely on stimpacks while in battle. You can either talk to them and use one of your stims to heal them via the companion wheel or put some into their inventory and trust them to use one.

Thanks guys thank you for your answers. I am playing in hard core mode, so I believe that companions can die when in hard core mode.