This Firefox extension enables you to create new custom search engines for the browser's search bar in a completely automated way.

Just open the webpage you want to create the new search plug-in for, right click on the keyword input field and select "Add OpenSearch plugin" in the context menu. A dialog will open for you to enter the new search engine name and description.

Once done, you'll have a new search engine to choose in your search bar menu and therefore you will be able to perform searches in your favorite webpages from any site.