It takes <1 hour to write a Firefox extension that modifies its UI in a trivial way, and that Firefox extension can be distributed easily. You can do almost anything by modifying the XUL and/or CSS, and there is a good framework and good documentation for doing so. The default Firefox UI is just a default. There are many extensions and themes out there that make it easy for even non-programmers to modify Firefox to look and behave the way they want it to.

While it's possible to modify Chromium's UI by changing the source, it's far more difficult, and, unless your changes are accepted upstream, you have to maintain and distribute a whole new set of binaries yourself. If you want to do something like make the tabs square, make the tab bar scroll, or replace the download bar with a panel or a separate window, you're gonna have a bad time. Chrome's "themes" are really just different title bar and tab backgrounds. If you're a non-programmer, you can't really change the Chromium UI at all. The UI defaults are not just defaults; they're the only option unless you want to write a shitton of code.

Safari and IE are closed source, so it's extremely difficult to modify their UIs at all beyond the meager options provided to you in the preferences, even if you're a rockstar programmer.

> It takes <1 hour to write a Firefox extension that modifies its UI

Not for anyone using Firefox. Browser software is not just for developers, in fact, the majority of the users is not developers. I think this discussion was from a regular users default point of view of the Downloads window.

I think I addressed this pretty well already. Read the whole post.

Non developers like my mom can extend Firefox to fix a better UI for downloads – and that's and argument for Firefox not to have great defaults?