Firefox,one of the popular web browser has many shortcut keys which makes the life of Web-surfer much easier. Below is a collection of one of most favorite of mine :  

Zoom in a Webpage               Ctrl Key with + Key

Zoom out a Webpage             Ctrl Key with – Key

Zoom Reset Press                   Ctrl Key with 0 Key

Open a new Tab                     Ctrl Key with T Key

Close a Tab                            Ctrl Key with W Key

Reopen a closed Tab              Ctrl Key with Shift Key with T Key

Move cursor to Address bar   Ctrl Key with L Key

Move cursor to search box     Ctrl Key with K Key

Scroll down the page             Spacebar

Scroll up the page                 Shift key with Spacebar

Refresh the page                   Ctrl Key with K Key (Good Ol’ F5 works too.)

Navigate to Back Page           Alt key with Left_Arrow Key

Navigate to Forward Page     Alt key with Right_Arrow Key

Search a keyword in page      / Key does the trick (Although Ctrl with F works as well)