Firefox doesn't really play in the Applescripting world. See [https://bugzilla.mozilla.org/show_bug.cgi?id=125419 this bug filed against it], and vote for that bug if you'd like to make it Applescriptable.

However, there are some workarounds for Firefox 2. Adium uses this:

if ((application processes whose (name is equal to "firefox-bin")) count) is greater than 0 then
     tell application "Firefox"
         if (count of every window) is greater than 0 then
               set the end of candidateURLs to «class curl» of window 1
          end if
      end tell
end if

Hengist says this might be written in appscript like this:

from appscript import *
ff = app('Firefox')
appscriptref = ff.windows[1]
aemref = appscriptref.AS_aemreference
aemref = aemref.property('curl')
appscriptref = ff.AS_newreference(aemref)
print appscriptref.get()

Unfortunately, [https://bugzilla.mozilla.org/show_bug.cgi?id=427448 Firefox 3 broke this workaround].