In Firefox 1.5, web feeds looked like this:

This tree view (with expandable/collapsible nodes) made it really convenient for viewing XML files (and RSS feeds). Fortunately, there is a way to re-enable this option in Firefox 2.

To achieve this, we’re gonna need to edit the FeedConverter.js file inside Firefox’s components folder. On Windows this should be located at C:\Program Files\Mozilla Firefox\components\FeedConverter.js. On a Mac (OS X) this should be located at /Applications/Firefox.app/Contents/MacOS/components/FeedConverter.js. Finally, on Linux, this might be located at /usr/lib/firefox-2.0.0.4/components/FeedConverter.js. (Note: this specific example is for PCLinuxOS 2007, and may vary slightly from distro to distro).

In the most recent version of Firefox, version 2.0.0.4, you’ll want to scroll down to about line 194 & find the following:

      // show the feed page if it wasn't sniffed and we have a document,
      // or we have a document, title, and link or id
      if (result.doc && (!this._sniffed ||
          (result.doc.title && (result.doc.link || result.doc.id)))) {
        // If there was no automatic handler, or this was a podcast,
        // photostream or some other kind of application, we must always
        // show the preview page...

        // Store the result in the result service so that the display page can
        // access it.
        feedService.addFeedResult(result);

        // Now load the actual XUL document.
        var chromeURI = ios.newURI(FEEDHANDLER_URI, null, null);
        chromeChannel = ios.newChannelFromURI(chromeURI, null);
        chromeChannel.originalURI = result.uri;
      }
      else
        chromeChannel = ios.newChannelFromURI(result.uri, null);

      chromeChannel.loadGroup = this._request.loadGroup;
      chromeChannel.asyncOpen(this._listener, null);
    }
    finally {
      this._releaseHandles();
    }
  },

Just change the “if” line (here it’s actually on 2 lines) to read:

      if (0) {

This code was originally found here, and updated slightly to reflect the updates seen in the 2.0.0.4 FeedConverter.js file.