You have this menu, not directly, but who offers the choice beetween 10 engines.
Other thing, instead of right-clicking, just click on search button, and it directly search the selected text with the default engine. As quick than a right-click.
But you can with a macro add or clear a menu entry. K-Meleon menus are easily customizable, I don't think Firefox can easier do it.
A+
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Web:
http://jujuland.pagesperso-orange.fr/
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc