During the configuration of Firefox on my iPad, I noticed that there is no option to add a custom suggestions API URL, like it exists under Android. Therefore, while using a custom search engine, search suggestions are missing. I think adding support would be a good idea to make custom search engines more useful/to bring them on par with default options.
While I know that there are technical limitations regarding Firefox on iOS, I don't think/hope that this is one of them.
The screenshot shows the menu under Firefox for Android. While the design is slightly different under iOS, it basically has the same functions minus the search suggestions API option.