with firefox you can right click and select check spelling from the context menu (the menu that pops up when you right click). Anything spelt wrong will be highlighted. The only issue is you might be using an American dictionary.
Also on that menu is languages hopefully you will have British English as an option. If not you can install a British dictionary, but it may be easier to stick with the American dictionary and a bit of common sense.
After all we should have have a reasonable knowledge of how words should be spelt even if Americans might disagree as to how they may be spelled.
blame him, I do