How to add a custom search engine to Firefox?

Is there a way I can add a custom search URL to the Firefox search bar? e.g. I'd like to provide a URL such as http://blahblah.com?search=%s, where Firefox replaces the %s with the content of the search box.

Both IE and Opera can do this, but I can't figure out how to set it up in Firefox.

Add keywords to the address bar:

For example, you can set up your browser to search for bread at stackoverflow.com by simply typing so bread into the omnibar.

Do this by right-clicking on the search bar on the website you commonly search. You can do this for the Search field on any website.

Specifically:

1. Go to the site you want to use to search
2. Right-Click on the search box you fill out on their page (not Firefox's)
3. Select "Add a keyword"
4. Enter the keyword to use when you want to search in that site (eg: "so")

I'd like to provide a URL such as http://blahblah.com?search=%s where Firefox replaces the %s with the content of the search box.

You can do this with Firefox! Add a bookmark with that URL, where %s is the search query, then simply set a keyword for the bookmark. You can then search using <key> <search term> in the address bar. For example, the bookmark http://www.google.com/search?q=%s with keyword g means you can type g stack overflow in the address bar to search Google for "stack overflow".

Alternatively, go to the site you want to search, right click in the search box and click "Add a keyword for this search".

The actual, no-bookmark solution for this task:

1. Open a web page that has a search on it (e.g. SuperUser: superuser.com).

2. In the address bar, expand the address bar by clicking on it.

3. If the website supports searching, it will show up in the bottom area of the expanded address bar. Click the plus icon that is enclosed in a green circle. This adds the web page’s search to the search engines in your preferences.

If you want to set a keyword (e.g. su) for this search, follow these steps:

1. Open the search preferences (or type the following URI in your address bar: about:preferences#search)
2. In the table under the column for “Keyword”, double-click in order to set your own keyword.