Interesting snippet filed for future reference. Thanks.
However, this doesn't really solve my problem which is using the -remote command from display :0.1 to open a URL in a new tab in an already running firefox on display :0.0. All I ever get is "Error: No running window found". I'm beginning to think it can't be done - does firefox implement the -display command properly - or at all? I'm just wondering because running
firefox -display=:0.0 -P default2
from a shell on display :0.1 starts a new instance on :0.1.
Anyway, as it's looking more like a firefox problem than a Slackware/firefox one, these questions would seem to be more appropriate for the firefox forums so I'll try there (again).