I've seen a couple scripts to open the current Safari page in Firefox, but none for the other way around. I use this with LaunchBar so I can easily view the page in Safari, then quit with a couple keystrokes. Here's the script:
tell application "Firefox"
activate
set FirefoxURL to properties of front window as list
end tell
tell application "Safari"
activate
set URL of document 1 to item 3 of FirefoxURL
end tell
[kirkmc adds: Works fine for me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070928171557619