Your mac is clean but Firefox eat most of your ram. You have only 4 GB of Ram in my opinion the best solution is to upgrade your ram of your mac at least 8 GB.
1. Check if you need all the Add-ons in Firefox. Disable all that you actually not needed.
2. There are some Log-In items that don't need at startup. They running in the background and eat many of the ram.
3. Open Apple menu -> System Preferences -> the click on Users & Groups
4. Choose your Account below current user and then click on the right to the Login Items
5. Remember which Login items you have and remove all unless ITunesHelper.
6. Choose Apple Menu -> Restart. Test If this solve the problem.
6a.) Open User & Groups preferences again and add the login items one at time and restart your mac. When the problem occurs again follow the steps above to remove the login item.
7. For cleaning caches boot into safe mode and login with your user account. Maybe the screen flashes it is normal and also the slower startup.
8. Open disk utility. Go to the Finder menu Go and below to Utilities. From there click on the disc utility tool.
- a) verify and repair your system drive
- b. check and repair disk permissions.
9. If all done restart your mac.
Check for updates
1. Open AppStore and click above on updates.
2. If you do not have done yet you should install the Lion Update
Also read the following articles:
You should upgrade to Yosemite if you want.