As far as I can see, the N800 browser stores bookmarks in XBEL format. At any rate, I've got a file /home/user/.bookmarks/MyBookmarks.xml in this format that contains all my bookmarks. XBEL stands for "XML [or eXtensible, by some accounts] Bookmark Exchange Language".

Many browsers including, I believe, Firefox either store or export bookmarks in Netscape bookmark file format (this typically has a .htm extension but the doctype declaration will be something like "NETSCAPE-Bookmark-file-1").

The two formats are similar in structure. In principle, it should be fairly straightforward to convert between them. There's a Python utility called xbel_parse that purports to be able to do the job. You'll find a man page here:

http://www.die.net/doc/linux/man/man1/xbel_parse.1.html

It looks a bit fiddly (apparently it reads only from stdin and prints only to stdout). I haven't tried it myself (I haven't even installed Python on my N800 yet) but it might be worth investigating if you have a lot of bookmarks (I don't, preferring to rely on autocomplete and the history list).

Not quite what you asked for, I know, but it might be very effective.