Firefox’s spell check defaulting (and reverting) to the wrong language

Check the Control Panel. Which languages do you have installed? If you have only Norwegian, try adding English as well:

You can then switch languages from the language icon in the taskbar.

You can also press ⊞ Win+C to bring up the Charms bar, select Settings / Change PC settings / General and modify the Spelling and Language options as required:

BTW, here's a complete list of supported spellchecking dictionaries and language/locales in IE 10.

Update: How do you type and spell-check in one language, but use the keyboard layout of another?

The problem with switching the input language is that normally the keyboard layout also changes, which is irritating. Here's what you can do. Say you have a German keyboard layout but normally type in English (input language), and want English spell-check (proofing language) as well.

Add English to the language list and make it your primary language by moving it to the top. Now click the Options link:

Here under Input method you can Add an input method:

So now you can have English as the input/proofing language but German as the input method/keyboard layout. You can subsequently press ⊞ Win+Space to switch between available combinations:

You can't change this on the fly, but you can change this as often as you like. There's two ways to get to this menu

1. Open Internet Options from the Control Panel or from Internet Explorer (Tools > Internet Options).
2. In the General Tab, click the Languages button at the bottom.
3. Click Set Language preferences.

Or:

1. Go to Settings in Windows 10.
2. Click Time & Language.
3. Click the Language side menu option.

Then, click Add a language, or if you've already added a language, you can re-order the list of languages you have chosen so that your preferred language is on top.

From here you can add whichever language you want. The reason I say you can't do this on the fly is that Microsoft Edge will need to be restarted before it will detect your changes.

To disable spellcheck, go to Settings, type "spell check" (without quotes) in the search bar and hit Enter, and then you will see the option to disable highlighting or autocorrecting of misspelled words. Again, Microsoft Edge will need to be restarted before it will detect your changes, if you have it open when you make these changes.