Search engines are added using the OpenSearch standard. When you visit a website and see a green circle with a + sign over the magnifying glass in the search bar in the toolbar, that indicates a website has an OpenSearch engine available to add to your Firefox search engine list (Managed from Options->Search->One Click Search Engines). Click the magnifying glass and choose Add "website". You can also visit http://mycroftproject.com/ for tons of OpenSearch templates.

(edit) I added this info to the ReadMe @ github for users unfamiliar with how Firefox handles OpenSearch engines.

(edit2) I'll look into a way of adding custom templates. I've been hesitant to implement anything like that because it would require maintaining a separate list of engines from FF's One-Click list, but it might be doable without getting too messy. Check the beta section in the coming weeks.

(edit3) v1.2.7 betas add the option 'Add Custom Search' to the right-click menu on input forms.