New Reddit Redirect от Erisa

This addon will always redirect Reddit to your preferred design when someone links you to forced old or new Reddit design pages.
Normal Reddit links are unaffected, it's only when someone decides to link you to a forced design URL that you are redirected.
The code this was forked from allows you to force Old Reddit, if that's what you're after then check it out: https://addons.mozilla.org/en-US/firefox/addon/old-reddit-redirect/