Was testing this the other day on my DA Power Mac G4. The keyboard (K780) and mouse (MX Master 2S) were both paired to a unifying receiver in Linux (OSS Unifying app) first. I plugged the receiver into one of the ports on my cinema display, and after a second, OS 9.2.1 recognized both. The K780 has built-in layout switching between Mac and PC, so I was able to switch to the Mac layout, and everything worked as expected. Scrolling and right-clicking on the mouse worked. I didn't bother trying anything else, but unless Classilla supports the standard keyboard shortcuts for forward and backward, I doubt the navigation buttons do anything.
Anyhow, just thought I'd share.