You are probably sometimes tried to save images or copy the contents of the website that forbid their use of the right-click. Most websites use javascript to prohibit the use of the right-click , so to enable it you should disable the javascript.
There are many ways to do ti , the simplest by my opinion is using Web Developer 1.1.9 , firefox add-on .

1) Downlaod Web Developer 1.1.9 from this link

2) Click Disable button

3) Select Disable Javascript

4) Again select “All Javascript” option.