Do you mean in the general page menu? If you right click on a tab itself you should get the close option.

you simply don't know what the context menu is.

right click anywhere on a page to bring out the context menu

Right-click on a tab to see the contextual menu with Close Tab.

You can use the Menu Editor extension to move menu items around.
https://addons.mozilla.org/en-US/firefox/addon/710

the-edmeister,

thankyou for your message

i know there are plenty of addons that can handle the job

i am in fact looking for whether there is a user setting capable of this

i found two other addons that can do the same thing.

https://addons.mozilla.org/zh-TW/firefox/addon/rights-to-close/

https://addons.mozilla.org/zh-TW/firefox/addon/closetab/

will check out the one you said in your message

'cause the context menu of my firefox is getting longer and longer !

Yep, the length of contextual menus is why I use the Menu Editor extension.