Rename it to "Bookmarks.html".

Change the Filetype/Creator from MSIE/TEXT to TEXT/MOSS.

Go to the Netscape Users/(Username) folder, in Preferences, open it, and there is a Bookmarks.html file. Stuff it in place (for a backup) and drag that new one to there.

Then go to the Netscape Communicator/Essential Files/Defaults folder, and drag a copy of that same bookmarks file to replace the one there. You're done. All your Favorites from IE are now your bookmarks in Netscape. You wll have to re-make a Personal Toolbar Folder and move your IE Toolbar Favorites to that folder if you use the Personal Toolbar feature.

Netscape -> IE:
You just need to make a backup of your MS IE Favorites file, then copy the Netscape bookmarks file, rename it to Favorties.html and changed to MSIE/TEXT in place of the original. Only the one location to worry about for IE.

You can also use URL Manager Pro to manage ALL your bookmarks for both browsers if you prefer. It's another app to use, sure, but it does a GREAT job of managing them.

For a different method of managing andusing bookmark files on the Mac, see caiseal's great tip here.