I had to do this a while back and it's nearly made me punch my monitor. The lowest circle of hell for the person/people who decided putting this basic functionality behind a fucking arcane flag. Few things make me more mad than having my finite life wasted hunting down solutions to problems that have no reason to exist.

Apparently its behind a flag because the feature is not completely ready yet. However I was unable to find what work is still needed for it to be considered ready: https://bugzilla.mozilla.org/show_bug.cgi?id=1106626#c18

Not ready my ass, this was a thing before deciding to move away from XUL.

In Firefox, there is a way that doesn't require a flag: create a bookmark with the url, and set its "keyword" field to the trigger word you want for the engine.

That works in chrome/edge too. (Even better, Edge matches urls with %s very good, and the URL bar offers past searches as suggestions better than Firefox does.)

They removed most bookmarks and search engine settings, but bookmarks with keywords work fine at the moment, for both mayor desktop browsers.

Also you can store them in a file(!) and import them for a nice cross browser experience, keeping track of them, and not losing them.

> Change `browser.urlbar.update2.engineAliasRefresh` to true

Just a note that this flag didn't exist in my Ffx 127.0b4. Created to True = surfaced the Add button as advertised.