I’ve not used i2p but I’ve had to mess with a lot of other random weird tools under bazzite. I’d suggest installing it in a distrobox. There is a command for linking programs from your host into the distrobox and then exposing them back to the host. I forget the exact syntax but I used it for vscode and intellij and it was really straight forward and worked well. I don’t see why it wouldn’t work with i2p and Firefox.