Tip:
Ever find yourself typing a search query into Firefox and then later updating it directly in DDG's search box and wishing Firefox would update its box accordingly? Well, here is how:
Download and install the
SearchBox Sync addon.
Restart.
Addons > SearchBox Sync > Options

Select Add...

Where the rule is:
https?://([^.]+\.)?duckduckgo\.com+/[^?/]*\?(.*&)?(as_)?q=([^&]+)
Et Voil
à!
Now your search box should sync with DDG.
This also works well when you highlight text, right click and select "Search with DDG".