$ Xvfb :99 &
$ DISPLAY=:99 firefox --display=:99 &
I, MOZREPL : Listening : 127.0.0.1:4242
$ DISPLAY=:99 xdotool search --onlyvisible --title firefox
4194416
$ DISPLAY=:99 xdotool windowfocus 4194416
$ DISPLAY=:99 ~/fillform.pl 
I, MOZREPL : Client connected : 127.0.0.1:46357 : chrome://browser/content/browser.xul
I, MOZREPL : Client connected : 127.0.0.1:46358 : chrome://browser/content/browser.xul
debug: going to mech->get()
Can't call method "addProgressListener" on an undefined value at /usr/local/share/perl/5.10.1/WWW/Mechanize/Firefox.pm line 574, <DATA> line 1.
I, MOZREPL : Client closed connection : 127.0.0.1:46358
I, MOZREPL : Client closed connection : 127.0.0.1:46357