Twitter has updated its Mac desktop software, the first time the micro-blogging service has refreshed the program since it launched last January.

Twitter 2.1 hit Apple's Mac App Store earlier Thursday; as of 4 p.m. ET, the free program was No. 7 on the store's bestseller list.

The desktop program now lets users display multiple timelines simultaneously, said Twitter in a post to its blog . Those timelines can be multiple user-controlled accounts, others' Twitter feeds, lists or mentions.

Each timeline appears in a separate window on the Mac.

Twitter also added new navigational commands, a separate New Tweet button, and redesigned the Direct Messages window.

Twitter's Mac software is based on Tweetie, an iPhone app that Twitter purchased more than a year ago from developer Loren Brichter.

Apple customers running Mac OS X 10.6, aka Snow Leopard, can access the Mac App Store from their machine's Apple menu. Users who have previously installed the Twitter client can grab version 2.1 by clicking the "Update" button at the top right of the Mac App Store display.