You could give https://addons.mozilla.org/en-CA/firefox/addon/gigablast-sea... a try.

If you don't want an extension, another option is to find it on Mycroft Project [0], choose Gigablast, and on the "Install plugin" page, right click the address bar and choose "Add Gigablast". Then you can set it as your default from the Firefox search settings.

[0]: https://mycroftproject.com/search-engines.html?name=gigablas...