I whipped up a quick tool to allow Firefox to integrate with Vista’s Common RSS Feed List. You can download and install it here.
After installing, you need to select “Tools | Options” in Firefox, then click the “Subscribe … using” radio button, the “Choose Application” button.
Browse to C:Program FilesMicrosoftCommon Feed ListCommon Feed List.exe. After selecting this, you can switch back to “Show me a Preview”.
Now every time you view an RSS feed, you will have the option to subscribe to it using the Common Feed List, which will make the feed show up in Vista Sidebar, Windows Live Mail Desktop, IE7, or any other client you have configured to sync with the feed list.
If you want to look at the source code, you can download it here.
When you subscribe using the Common Feed List, you are using the RSS engine built-in to IE7. Feeds subscribed in this way save your machine bandwidth, since only one copy of each feed is downloaded even if multiple applications use the feed. You also get multi-machine sync of feeds and read/unread status, and ability to read your feeds in the cloud or on Macintosh or mobile for free.
IE7 on Windows Vista runs in a “sandbox” called Protected Mode, which limits the amount of damage that can be caused by in-process exploits. Since protected mode is an operating system level constraint applied on a per-process basis, protected mode does not apply to IETab running in Firefox on Vista. Therefore, if you want Protected Mode, you should use IE7, which includes the subscribe experience shown above by default.
Update: The downloads linked above have been updated to support Firefox 3 (Thanks, Warren!). The code should work on both Vista and Windows 7. If you need the old version that works with Firefox 2, you can get it here.