Why is my MacBook Pro slow? (2010 model)
Aside of getting more RAM, you can do following.

Disabling the flash in your Firefox will save you some.

https://addons.mozilla.org/en-US/firefox/addon/flashblock/

To your third question:
On my OS x 10.8 the "kernel" alone uses 1 gig already!! so you are definitely running short on RAM. 

Just get another 2 Gig and see it fly.