Firefox uses a lot of memory when you have more than one open tab.
How to fix it? Easily, just follow these steps:
1. Write about: config in the location bar (where you usually write the address of a website) and then press enter
2. Right click on the window that appears
3. Choose “new”>”integer”
4. Copy in the new window: browser.cache.memory.capacity
6. Set the value to 60000
7. Restart Firefox