Rated 5 out of 5 stars

Firefox has a long-standing bug where the user's spell check language preference is not honoured.

A solution exists for Pale Moon users:

about:config - spellchecker.dictionary.override;en_GB

More info, and detailed instructions with screenshots:

http://blog.spiralofhope.com/?p=6792

This review is for a previous version of the add-on (2.25).