I do not think that is a good idea. If I download something, I like to
keep an occasional eye on the progress of the download. I do not want
to have to keep switching tabs to do that. I want to stay on the same
tab to keep my place while reading a page. If the bookmarks library
became a tab, how could you drag a bookmark into it? With bookmarklets,
that is a handy way to bookmarks those.