Feed Proxy is an attempt at working around Firefox 2.0's feed preview, which while generally nice and user friendly, gets in the way when trying to debug feeds. The script will fetch the feed and add 512 bytes of padding before the start of the content, so that it bypasses the content sniffing that Firefox does.
To use it, click on the "Register" button below to add Feed Proxy to Firefox's list of services that feeds can be sent to. Then, when viewing a feed you can use that option from the drop-down to view the raw feed source.
The caveat with this solution is that the feed is re-fetched by my server, so no cookies or authentication data is passed through, which may make this less than ideal for GData and other authenticated API feeds.