i use the tab key to switch between panes in the mailnews client. i find it irrating that it takes 6 presses to complete full cycle. it should take only 3. note that this report assumes you are displaying the Mail Toolbar, the Search Toolbar, and the Message Pane. My proposed fix is to skip the Compose button and both spots in Search toolbar while tabbing.
If there is interest from others, I'll file a feature request on this.
