well i always say that opera is better than firefox because of the resources firefox takes but i will now tell you a trick to make firefox lighter and work like opera!!
follow these steps:
1)open firefox, type about:config in the address bar and press enter
2 right click on the page and select new option then select Boolean
3 enter the following string config.trim_on_minimize press enter
4 find the value and make sure that it is set to true
5 restart firefox,

what this has done is when the user minimizes firefox, it will reduce its memory consumption.
enjoy