If you have tried to use Thunderbird as the default feed handler in Firefox, you should have realized it doesn’t work. It’s a shame, yes. But luckily, alta88 has figured it out, and Chris Ilias has written a simple article to fix this in Firefox.

Once you have done all changes specified in the article, remember to create a News and Blogs account in Thunderbird before subscribing to a web feed in Firefox. To do this, in the Tools menu, select Account Settings…, press the Add Account… button and select RSS News & Blogs.