Firefox uses a lot of RAM. It is the number one complaint about this otherwise awesome web browser. So what can you do to solve this issue? Well there are a few rumored fixed out there in the wild.
Here is the solution to the problem:
1. Open up about: config by typing it into your Firefox address bar.
2. Right-click, go to new and then integer and name it this: browser.cache.memory.capacity
3. For the integer value, enter in 15000 if you have 512MB to 1GB of RAM. For those with less (between 128MB to 512MB), try a value of 5000. You're done!
Then all you should need to restart Firefox for it to take effect.