How to add a custom search engine to Firefox?

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".

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")

The Add custom search engine extension lets you create a new search engine and customize it.