booksync 
comment #5 raises a very good point: why have to click to up/download my bookmarks?
so here is v2 of the booksync script:
it's just the same script, but with a little change: it downloads your bookmarks when you open mozilla, and uploads them when you close it.
to use it, all you have to do is download it and read the instructions in the file.
20030521 - Update
fixed the issues mentioned by Eule in comment #6.
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