Confirmed this behavior - in an stock install of Jaunty that is (according to System->Administration->Language Support) set up for US/English, Firefox thinks 'behavior' should be spelled 'behaviour', 'artifact' should be spelled 'artefact', and so on.
In FF, Tools->Add-ons->Languages shows that en-gb is installed
Confirmed this behavior - in an stock install of Jaunty that is (according to System- >Administration ->Language Support) set up for US/English, Firefox thinks 'behavior' should be spelled 'behaviour', 'artifact' should be spelled 'artefact', and so on.
In FF, Tools-> Add-ons- >Languages shows that en-gb is installed