This addon automatically switches between the installed dictionaries for spell checking in text fields. It does so by looking at the last 10 words of text and picking the language that results in the least spell checking errors.
At the moment this addon does not have any configurable options. I'm looking forward to suggestions what you would like to see configurable. If you want to temporarily disable this addon, you can do so at about:addons — disabling/enabling the addon does not require firefox to restart.