A couple of weeks ago my PC crashed and all Firefox profiles were deleted, including bookmarks. This wasn't a massive problem in this case as I'd backed up the most important ones, but I've heard of other people who have had similar problems when using Firefox.
So I was interested when I saw Jane Hart mention Zinkmo on her blog. This is a piece of software that you install on your PC (not Mac) and it allows you to syncronise your bookmarks across multiple browsers and to access them online if you aren't on your own computer.
Now my bookmarks are the same in Internet Explorer, Firefox and accessible online - so I can't loose them again! However I noticed that the software at times runs quite slow - and other people have noted this too, so you might want to wait for the software to develop before you install it?