Works just like ChromeIPass on Google Chrome, thanks to WebExtensions. I've been using this as a PassIFox replacement since Firefox Quantum, and I must say that this addon works even better. It has a built-in password generation form backed by KeePassXC (or whatever KeePass GUI you're using locally) and a nifty in-browser interface with lots of settings. Overall, a worthy replacement for the legacy addon.

Side question: why is the addon's icon missing from the Mozilla Addons page? I was initially wary of installing this addon because of the sparse description, no screenshots, and lack of a dedicated icon, which gave me the subtle impression it was unfinished or perhaps unmaintained. In fact, KeePassHttp-Connector is actually quite attractive interface-wise. I regret not installing it sooner.