http://zzzmirror.i2p/topics/2997-firefox-tricks-that-may-save-on-user-effort-desktop
I would expect that FoxyProxy and non-FoxyProxy users are easily distinguishable from one another by just adding a clearnet link, if it tries to go though false.i2p then the user doesn't use FoxyProxy, if it goes through Tor then you know that they are using FoxyProxy. A fingerprinters dream? An alternative might be to setup two Firefox "Profiles"? One that is i2p only (NO TOR, that's what Tor Browser is for), and another that is "clearnet" only.