Is it possible to have Mail.app open and manage messages using tabs instead of windows? Google tells me a plug-in called MailTabs was created for this back in 2011, but it's now defunct, and I can't seem to find any other options. Thanks!
MailTabs previously would only install up to Lion. I just confirmed MailTabs can now install on Yosemite, since it seems to be detecting 10.10.1 as 10.1. Messages that are opened as separate windows will fill the window as if taking up a tab. The Shift-Command-LeftSquareBracket and Shift-Command-RightSquareBracket keys work in that you can navigate between the messages and the normal app view, but there are no actual tabs present. The messages do not stay open between closing the app and re-opening the app. The full-screen view is very broken, and it even got stuck for me, leaving me only with the option to Force-Quit.
This plugin would be extremely useful if it was revived, as it can give you the tab functionality available in Thunderbird. The MailTabs page on the site has not worked for a while and now the parent website itself for http://simplicity.fm now seems to be down as well. My hope that this plugin will be available again is slowly dwindling because of this.