It's infuriating that the default setting for Firefox is to double space paragraphs when you are responding to posts in forums. Here is the fix...
Type about:config in the address bar. You will be shown a warning letting you know that you can really screw up your browser messing around with these settings.
Type editor.use in the search bar and you will see a setting for "editor.use_div_for_default_newlines"
Change the setting to false, and your responses in forums will no longer be double spaced.
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