As far as I know, you can only automatically bookmark in Explorer. The jscript involved is a proprietary (non-standard) method from Microsoft called AddFavorite(). Microsoft reference [msdn.microsoft.com]
The rest of any script offered online is usually devoted to generating a "friendly reminder to bookmark" messages for any visitor not sniffed out as Explorer, and if you do this, sniff very well, since User Agent spoofing is common today.
In general, I think MS made an error exposing Favorites to jscript like this and I hope it gets patched. Lots of unscrupulous sites don't ask for user action first, but use script that install their bookmark in a stealthier way. Bogus, IMO.