Over the last few years, we've seen the Mozilla family of browers  evolve into feature-rich, usability-oriented browsers, especially with the new concepts of tabs and extensions, the latter allowing, amongst other things, new side pane applications.
This differs markedly from the traditional "one app, one function" mantra of the Unix community, although there are parallels with the history of the editor Emacs, which some say is now an operating system in itself.
From a Linux perspective, it may seem like a mistake to embark on including tab functionality within the application rather than contributing to existing window manager projects that allow tabbing. Will the Linux community thus come to suffer from a Windows legacy in Firefox?
Secondly, the idea of side panes can also seem short-sighted under the same considerations. While some side panes provide functionality that is clearly part of the browser, others do not, and other parts of Firefox that could fall under the side pane paradigm are not implemented in that way: for instance, the download window.
A compromise would be to rebuild the framework such that the user could decide whether a given app should run in the sidebar or an external window. However, the main browser window menu does not interact with sidebar content except to determine what should be shown in the sidebar. Hence, the sidebar could always be a separate window that the window manager could be instructed to always place next to the browser. The simplest way to deal with switching between different sidebar views, e.g. history and bookmarks, would be to simply spawn a new window. Hence, different sidebar windows could be shown as tabbed.
I conclude that having "Downloads" as a separate window is a step in the right direction, and that this should be applied to all extensions. The focus must then be on contributing to a window manager that supports the above features. The one I am thinking of is Ion ; there may be others with similar features.
 Mozilla, Firefox, Epiphany, Galeon, Camino, ...
I have posted the same article on mozillazine.org; perhaps it is best to discuss it there: