For search engines that could be recognized by Firefox when the page is visited, please include an "add search engine" item in the menu.
The TL;DR:
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.