I hope that Firefox's remote control feature is reasonably well known, but I suspect most people don't know about the surprising and relatively weird effects it has in the Unix version of Firefox; certainly it surprised my co-workers when they stumbled across it.

(Remote control is the feature where if you already have a running Firefox and try to start another one, it will just open a new browser window (or tab) in the first one.)

The root of the surprising behavior comes from how Firefox's remote control works. Rather than use a conventional IPC mechanism (Unix domain sockets, for example), instances of Firefox communicate with each other through X properties. Because X properties are not a per machine thing like Unix domain sockets, the Firefox remote control is global; a Firefox on a machine that you've ssh'd in to can remote control your local Firefox (and vice versa).

(This is what surprised my co-workers; they expected it to be a per machine and per user thing.)

The default Firefox setup on Unix is quite insistent on using remote control if at all possible, to the point where it is impossible to start two copies of Firefox, even on different machines. This can periodically be annoying, for example if you really need to do some particular browsing from a specific machine but don't want to shut down your regular Firefox session.

(It can also be puzzling if you don't realize what's going on; you might find that downloaded files aren't where they're supposed to be, or that some machine's web-based control interface just doesn't seem to be responding.)

Fortunately this behavior is all in the firefox wrapper shell script, which you can modify to get around the issue; see the check_running function and where the ALREADY_RUNNING variable gets used. Note that having more than one Firefox running will make any remote control stuff you do potentially confusing, since you don't know which one will get remote controlled.