Previously I blabbered about how writing themes for Firefox sucks.
Now, on a related note: why is the Opera theme browser so much superior to Firefox?
In Opera the theme browser opens in a window in the application itself. You browse some themes. You pick one and click Download and it downloads right in the window there. It applies (without restarting the browser, note) and asks if you like it. If you don't, then it reverts back to the original theme.
Changing between already-installed themes involves a SINGLE CLICK in the themes dialog. You click a theme name and it applies instantly. That's it.
When browsing/applying themes, Firefox opens up this window. Clicking the obscure "Get More Themes" text link takes you to an external URL, https://addons.mozilla.org, in my opinion one of the worst-designed theme sites I've ever seen. There, you get to browse through text links of themes on the main page (without thumbnails). Sometimes thumbnails appear on the secondary individual theme pages, and sometimes not; but not a thumbnail in sight on the main page. This means to visually browse themes (judging by, you know, what they look like rather than their name and version number), you have to click about a hundred times on all the theme links.
Once you find a theme you like, you can click the "Install Now" link on that page, in which case a dialog box appears asking if you want to install a theme from this JAR file:
Anyways. Clicking OK to install the JAR file makes the theme magically appear in the original (separate) Themes dialog I posted a screenshot of above. And then, to try the theme, you double-click it or click Use Theme, and THEN you have to restart your browser. And if you don't like the theme, you have to restart your browser AGAIN to change the theme back.
Firefox used to let you update your theme without a browser restart. This was years ago, but I remember it. And it never worked right. Sometimes parts of the interface would change and parts wouldn't. And you'd have to restart the browser to fix it (or to discover that the theme just hosed your Firefox profile and you need to wipe it and start fresh).
Firefox got this wrong in pretty much every way. I just tested Firefox Beta2, and other than combining the Themes and Extensions windows into one window, and moving some buttons in the Themes window around, it looks exactly the same. Bad bad bad.
(IE has no themes. So it still sucks worse than both. apply "IE_Sucks" tag)