This is the best I could do:
name this script firefox.scpt and save it anywhere:
tell application "Firefox"
set myFirefox to properties of front window as list
set currentURL to item 3 of contents of myFirefox
return result
end tell
assign a trigger for this script:
do shell script "osascript /Users/chris/Desktop/firefox.scpt | qs"
ofcourse, you have to replace the location in this script.
You can set the trigger Cmd+Control+P for this script. After pressing
this hotkey, you can press Cmd+Shift+P unless "P" brings up something
other than "paste" in your actions pane.
(Please see: the first script sometimes (very rarely in my case but it
depends) fails to get the
firefox URL and this is a firefox issue. A restart of firefox fixes
it. This is a known issue: http://getsatisfaction.com/rescuetime/topics/firefox_3_on_mac_still_not_supported
and
http://www.dmahaffy.com/2008/03/firefox-30bx-and-applescript-why-is-it.html)
