I think I found the solution. Ok, not to the whole problem, but at least to the bookmarks issue. And the solution is another Firefox extension (add-on), and it is named Foxmarks.
Its features are much the same of Google Browser Sync (i.e. it's not a backup- but a sync-mechanism) and Bookmarks Online (i.e. it works)... together! No, it's even better, since, other than some additional features, I found it also solves my Sage to-be-read problem! And this is by far the more valuable feature for me! It solve the problem because, I found, Sage store the "visited" info into a crypted string inside the Description field of the bookmark properties: so, taking synchronized this string like Foxmarks do is all I need!
The other features (maybe you are more interested in) are the following.
The synchronization is (could be) automatic (i.e. periodic) in the background (you have not to remembr to sync when you think you will change PC). Moreover, the synchronization mechanism needs not to be logged on only once (like Google Browser Sync does instead). So you could make Foxmarks synchronize bookmarks for more than a computer in real time (or so...): as you can read from the FAQ, bookmark changes you make are typically made available to other computers within 5 minutes, and those changes are automatically synchronized into every other computer within 60 minutes, or sooner if you synchronize manually.
Foxmarks also lets you access your bookmarks from any computer anytime via my.foxmarks.com, so that you can even access your bookmarks from any internet cafe' or any friend's computer!
So, for me (and for the bookmarks issue only, alas...) this solve the problem of join together the ubiquity-feature of webapps, with the even-offline- and right-in-my-hands- features of local apps!
PS
Edo, Foxmarks also has a feature for you: you can make it to use your own server! Enjoy!
