Slow Computer With Firefox?

I just bought a new computer, Dell 740 and refurbished.
I don't know enough abut computers but I asked the sales guy at a factory direct if this computer was good for just internet. it only has 2.3 GHZ and he said it would be good. another computer I used to have was 3.2 GHZ and I don't know if that made a difference, but it didn't seem as slow for pages/tabs to load as this computer.
today on the internet for the first time with this computer pages/tabs seem slow. I changed different things with Firefox to make it faster through about:config.
I was looking at CPU for usage and with twitter and other tabs open to would go right to 100% and pages/tabs were slow.
is this because of the 2.3 GHZ?
or maybe Firefox itself?
is there a way to make Firefox faster other than a different computer?
would another browser be better/faster?

 

It would help if you can post your computer specs.

 

Open Task Manager, click the CPU column heading and look at the processes near the top of the list. Although the list keeps changing it should be possible to figure out which processes are hogging the CPU. Post a screenshot if you can.