I recently upgraded to the new 1.5 beta versions of both Firefox and Thunderbird. So far, I haven’t encountered any real problems. The only thing I regret about trying the betas is that most of my extensions don’t work anymore. I’m hoping that the extension authors will update soon.
The new version of Thunderbird has added improvements to the RSS handling, so that hacks aren’t needed anymore. You can import and export feed lists in OPML format, and you can even have sub-folders now — sortof. You can add multiple feeds under a single folder. But the feeds won’t be individually selectable — the items all just intermingle together under the folder. There’s also no auto-detection of feeds, or drag-and-drop, both of which would be really good additions.
For more information about what’s new in the Firefox beta, see Simon Willison’s Firefox 1.5 Developer Highlights.