If you spend hours in the internet and sometimes lazy enough that u prefer to hold pees rather than going to toilet, it's a must have tool for you :
Install firefox plugin mouse gestures
Click and hold the right mouse button and drag left, it will go back to last page.
Some examples:
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Opens the link crossed by the gesture in a new tab.
If there is no such link, an empty tab is opened.
Install mouse gestures 1.5.2 (Open this link from firefox)
( I kinda advertised it... coz i loved it )