If you're using multiple computers, e.g. one at work and another at home, chances are, that you more than once have been searching for a bookmark - only to realize, that you bookmarked the site on "the other computer".
If you can say "Yup, tried that!" - then what you're looking for is Foxmarks. It's a tiny (158kb) Firefox AddOn which you install on each computer, and it will work silently in the background to keep your bookmarks synchronized. As a bonus, log in to my.foxmarks.com from any computer anywhere to access your bookmarks.
Having used this AddOn for a weeks or so, I simply can't imagine life without it .. as I saw on the download page: THE reason to use Firefox - well, I have to agree.