Here's the stream view grouped by day:
And here's the period selector in the river view:
The second preview release also includes a number of enhancements from alta88, who has added several layout and header options to the list view along with a bunch of other improvements (like a customizable toolbar button for quickly accessing Snowl features).
Here's the menu for selecting those layout and header options:
Still to come is a lot of behavior and visual design work, a number of additional bug fixes, and several more features, including Twitter-based message sending.
Get Snowl for FirefoxGet the preview release: Snowl for Firefox.
Warning: this version is a primitive implementation with many bugs, and subsequent versions will include changes that break functionality and delete all your messages, making you start over from scratch.