This article Reducing memory usage - Firefox is a good reference on how to reduce memory usage by Firefox. Follow through the suggestions in this article will sure make your Firefox much faster.

Major points:

• If you don't need all the extensions and plugins, you can start firefox at safe-mode
• Clear download history
• Configure memory usage limit
• Restart Firefox periodically. To help keep the sessions upon restart, using Session Manager Firefox Addon.

To start firefox in safe-mode, just do

$ firefox -safe-mode &

More references:

• How To Reduce Firefox Memory Usage
• Reduce Firefox Memory Consumption