I like to use custom search engines. Firefox unfortunately does not have an interface to add them. In fact, they recently even dropped the API for programmatically adding them.
The suggested workaround is to create an addon that adds your custom search engines. Unfortunately, one addon can only add a single search engine. That doesn't exactly scale well.
This leaves OpenSearch descriptions as the only sort-of viable method. So, here are my custom search engines as OpenSearch descriptions.
Note: These search engines probably aren't useful for you. Feel free to try them, though.