Firefox browser future version allows removal of search engines using the delete keyboard shortcut. Present Firefox lets you remove search engines listed in the Search section of preferences page when selected,¬†using the ‘Remove’ button.

Firefox 50, currently in Nightly, allows to remove the selected search engine using Delete keyboard shortcut. You can restore the deleted search engines if you want by clicking on ‘Restore default search engines’ button.

Firefox 50 search preferences page