You don't actually need a third-party utility...all you have to do is enable the Safari debug menu. Type the following in Terminal:
% defaults write com.apple.Safari IncludeDebugMenu 1
Restart Safari, and you'll see the Debug menu. You can then export your Safari bookmarks as an HTML file.
Then in Firefox, just import from the file.
BTW, if you want to turn off the Debug menu in Safari, replace "1" with "0" in the above command.