I've been using mozilla email clients for 9 years now over which time they haven't changed much, and this new release of thunderbird is no exception. The most significant thing about this release is that mozilla have directed some resources towards the email client which has until recently languished without much care and attention.
thunderbird vertical layout

The screenshot above shows thunderbird in a vertical layout which I've decided to try in conjunction with this new version, as previously I used the "classic" layout. You can see in the left pane that both my email and news feeds are handled by thunderbird, which includes my twitter messages as displayed in a per folder customized view below.

thunderbird twitter display

Installation

I upgraded from thunderbird 2 but unfortunately there were no v3 packages available for my linux distro, and mozilla themselves don't release linux builds in standard linux packages for some reason. They should should take a leaf out of Google's book here and look at how they release the chrome beta for linux, which coincidentally was released on the same day as thunderbird 3. I extracted the thunderbird release tarball to /usr/local/thunderbird/ to supersede the official one for my distro. Upon running it though I was disappointed to find it didn't pick up my existing mail and settings. Some investigation revealed that ubuntu use ~/.mozilla-thunderbird/ as the profile directory rather than the standard ~/.thunderbird/, so after renaming that directory I was happy to see all the old settings honoured. I also notice there will be improved migration from V2 to V3 in V3.1. As for plugins I only use a couple. The existing "Remove Duplicate Messages" plugin was not compatible with the new thunderbird, but there is a new version which is, and is faster and less buggy to boot (the old one intermittently crashed the whole client).

Improvements

Not much to report here TBH.

Issues

There is nothing to preclude me from using the new version but there are a few issues. [Update: Sep 2012. I updated from thunderbird 6 to thunderbird 15 on my Fedora 15 distro, but immediately hit a huge issue with stalls when scrolling the folder pane. I "upgraded" to thunderbird 17 in Feb 2013, and the stalls actually got worse. I downgraded back to thunderbird 6 without issue.]
© Jan 9 2010