For search engines that could be recognized by Firefox when the page is visited, please include an "add search engine" item in the menu.
Adding another search engine that isn't on the predefined list isn't as simple on my mobile devices as it is on my desktop. On my Android devices, I need to know what the search string is somehow, in order to add a search engine manually.
On desktop, I could just click a button in the drop-down of the search/address bar after visiting the search page, and it would be added automatically. That sort of thing seems much more intuitive and pleasant for me, as a browser user, than having to key in cryptic strings of characters. I'd like to see a similarly simple to use feature on mobile.