I had some initial trouble getting this to work with my Firejail configuration, but that's not terribly surprising. It's connecting over STDIN rather than HTTP, and an overzealous sandbox is going to have some trouble with that. I think part of the problem might have been that I was blocking the /tmp directory where the KeePassXC socket lives, so if anyone is having the same niche issue, try unblocking that.
Anyway, my several minutes of sandbox fumbling aside, this has been a great successor to PassIFox and KeePassHTTP-Connector so far. I love that KeePassXC now has a "match URL scheme" option, so I would have passwords filled on say, "https://google.com" and not "http://google.com", and I appreciate that it doesn't autofill by default and tends to require me to click a bit in the username field first, unless I configure it otherwise. It's an ideal balance between convenience and security. I hope to see this add-on carry on for years.