I use Webkit, which you can get here,
http://webkit.org/, which is the open source Safari that has new builds daily.
There is no open source Safari. Safari uses Webkit. All that happens is the Safari team chooses a recent, trouble free build of Webkit to integrate into the latest version of Safari.
Mail has data detectors, which is a major plus.
Mail handles multiple accounts nicely, with its unified view but also the ability to see just the messages from one account.
Smart Mailboxes are nice (although Thunderbird also has saved searches) and the stationery in Mail is very good too, plus you can make your own if you choose.
If your used to Thunderbird and very happy with it then stay with it, if you have used neither and are looking for the first time then I believe Mail is best at this point in time.
Safari vs Firefox has been done so many times. It comes down to this:
Safari has the better rendering engine (faster, advanced feature support (CSS, HTML 5 etc.) more portability), Firefox has the add ons, skins and the cross platform appeal.