When I go to Hamburger > Add-ons, I see this (after clicking on 'Dictionaries' down the left-hand side):
http://upl.co/uploads/Dictionaries1516253126.jpg
If you're not seeing 'Dictionaries' it suggests that you're using an old version of Firefox, as dictionaries used to be listed under 'Extensions'. (Look there to see if you've got a British English dictionary listed).
To check which version of Firefox you're using, go to Hamburger > Help > About Firefox. (You should have version 57.0.4)
To install a British English dictionary, go to Hamburger > Add-ons and type 'British English' in the search box. From the results which appear install 'British English Dictionary (Marco Pinto)'.
When I go to Hamburger > Options I see this (after checking that 'General' is selected and then scrolling down):
http://upl.co/uploads/Spellcheck1516253562.jpg
It's the box at the bottom of my illustration which needs to be ticked.