It's the Add to Search Bar 2.0 add on.
If a site that you regularly use has a search bar function to it, with this add on you can add that search to the search bar on your browser.
For instance, let's say you want to add the search feature from the Lin Yutang's Chinese-English Dictionary of Modern Usage - 林語堂《當代漢英詞典》網絡版.
After you've installed the add on from Mozilla. You go to the web page and right click on the search bar.
Once the search function has been added to the search bar, all you have to do is highlight any character or character combination to look up in Lin Yutang. It'll pop open a tab with the definition page.
It'll work for most sites that have a search feature but not all.
Here are some sites with search functions that you might like to add.
Hanyu Da Zidian
Lin Yutang's Chinese-English Dictionary of Modern Usage
Unicode's Unihan index