>>>english u.s the same as uk,
No it most definitely is not!
If you click on 'Systems & Languages' (underneath the big 'Download' button) you end up here:
http://www.mozilla.org/en-US/firefox/all/
That allows you to select the 'English (British)' version.
As you seem to have already installed the US version, Tearinghair, I strongly recommend that you should now change the dictionary over to Brutish English.
To do so, go to Tools > Add-ons.
Enter 'British English' in the search box.
Click on 'Install' alongside 'British English Dictionary'
(You might have to restart Firefox after doing so. I can't actually remember).
Then come to any question on AB and type 'colour color' into the answer box (obviously without clicking 'Submit'!).
If 'color' is underlined in red as being incorrect, the British dictionary is already active and you don't need to do anything else.
However it's more likely that 'colour' will be underlined in red, meaning that the US dictionary is still active.
If so, right click on it and go to 'Languages', clicking on 'English (United Kingdom).
Then the British dictionary will be used thereafter.