Well, I tried

"C:\Program files\Mozilla Firefox\firefox.exe" -remote openURL(http://
+google.be)
[download]

on Windows with Firefox 1.5.x, and it did properly open the requested in a new tab. So things might have improved...

However, the requirement for the full path is very impractical. Firefox is a GUI program, and there's really no need for it to be in PATH. So typically, it isn't.

And, there appears to be no need for the arcane command line syntax, just an URL as an argument seems to work too.

"C:\Program files\Mozilla Firefox\firefox.exe" http://google.be
[download]

update: Corion, who is lucky enough to have Firefox as his default browser (corporate policies, meh!), says the command line in the registry looks like

"C:\Program files\Mozilla Firefox\firefox.exe" -url "%1"
[download]

%1 is the command line argument, either an URL or a file path, and the quotes around it are just for any spaces in a path of a html file. So there you have it, the officially recommended command line.