For anyone joining this thread late: Search suggestions are supported in Firefox for Android since sometime in 2023. Just be sure to edit your search engine and set the suggestions URL to "https://kagi.com/api/autosuggest?q=%s" as specified in the Kagi docs, then enable the "Show search suggestions" option if it's not already on.
Currently, you can find these options under Firefox > Settings > Search. Firefox for Android's search widget works great with Kagi.