In Firefox custom keywords are just bookmarks, and those sync. So this works on mobile. All you have to do is create bookmark with "%s" in the URL where you need substitution and give it a keyword, and typing `<keyword> <searchterm>` in the URL bar will use the bookmark .
Unfortunately the UI isn't too great -- on desktop, the suggestions between the awesomebar show you that a keyword search will happen (just like when you type something matching an open tab it tells you that it will switch to tab unless you hit the down arrow to choose something else). On mobile there is no such indication, but hitting enter after typing the keyword followed by the query still works fine and does the keyword search thing.
Thanks, I wasn't aware of this, particularly as the "normal" search engines in Firefox don't sync. I think this is in the works but it's been held up by some kind of issues with their search partner deals.
Unfortunately the UI isn't too great -- on desktop, the suggestions between the awesomebar show you that a keyword search will happen (just like when you type something matching an open tab it tells you that it will switch to tab unless you hit the down arrow to choose something else). On mobile there is no such indication, but hitting enter after typing the keyword followed by the query still works fine and does the keyword search thing.
I might try to fix that.