If you set it as default search engine, it works easily with sharing with the firefox app. But then you have to switch search engines for every time you search something else in that app.
Another workaround could be to install another firefox fork to only use it for this search. You can set up your intended search there and then only use that app for sharing search terms to it.
You can do that with Fennec, Mull and whatever else is out there.