Why would I do such a thing -- go back to Icedove (aka the Mozilla-coded Thunderbird e-mail client) version 3.0.11, which shipped with the now-aging Debian Squeeze, after months of using version 5.0 from the Debian Mozilla team APT archive?
A number of things -- some pertaining only to me and my workflow, others more general -- made me revert to the older Icedove on my main Debian Squeeze laptop.
Here they are in no particular order:
While Icedove/Thunderbird 5.0 is certainly a step up from 3.0.11, it's not anywhere near what Mozilla is shipping now, which is version 10.0. Why are the Debian Mozilla developers doing so well with Iceweasel/Firefox, which is at version 10.0 in their repository (and on my system) while letting Icedove stall at version 5.0? I appreaciated the added "search this folder" capability of post-3.0 Icedove, and I have made a lot of use of it. But along with added functionality, some went away ...
Extensions of all types don't seem to work with the Debian Mozilla team's Icedove 5.0. I can't use Iceowl/Lightning because the old version in Squeeze no longer works with 5.0, the versions of the extention out in the wild also don't work, and there is no version of Iceowl/Lightning in either the Debian Mozilla archive or Debian Backports proper. I miss this extension. I had been using Evolution as a backup mail client and calendar, but the performance of Icedove/Thunderbird is so far superior, that I prefer to use one mail client only, and I'd like that client to have calendar capability. Also, while it's quite possible that I will remain a GNOME desktop user for some time to come, I could also switch over to Xfce, LXDE, Enlightenment or other, and I'd rather stick with an application that isn't tied so tightly to a single desktop environment as Evolution (though there are many other GNOME applications I use so heavily that the chances of me sticking with GNOME -- even GNOME 3 -- are very good).
None of the Google extensions for Icedove/Thunderbird (sharing contacts and calendar) work with the Icedove 5.0 from the Debian Mozilla archive. Now I freely admit that I never found a good contact-syncing extension for Icedove-Google (i.e. one that didn't mangle the contact fields), but at least having the option to try one or more is better than them not working at all.
Aside from the "search this folder" feature, I really don't have a problem with the feature set or performance of Icedove 3.0.11. I'm happy to either wait for Wheezy for a new Icedove, or upgrade/reinstall to Debian Wheezy right now.
Using Backports like a sledgehammer -- i.e. using every available Backport -- isn't the way to go. It's best to pick and choose the Backports and outside packages you absolutely need. I don't feel Icedove meets that criterion -- especially when the package hasn't kept up at all with upstream at Mozilla. And with so many restrictions on using Icedove 5.0 with other Debian Squeeze packages, I'm actually getting more functionality by reverting to the "native" 3.0.11 Squeeze package than using a newer Backport that is already extremely old when compared to the newest version of Icedove for the platform from upstream (aka Mozilla).
At the risk of repeating myself, unless you absolutely need a newer package on your Debian machine, sticking to what the distribution ships almost always means less trouble than forging ahead with newer applications from any number of outside sources. While I've never regretted "jumping ahead" to the newest Iceweasel from the Debian Mozilla team, I've been less happy with Icedove; unhappy enough to revert back to a package that I feel works better with the entire Debian system.
A few hours later: There seems to be no performance penalty in Icedove 3.0.11 vs. 5.0. I have yet to install any extensions (which I will do from the Debian Squeeze repositories).
A couple of hours after that: I installed iceowl-extension, and it's syncing perfectly with my Google Calendar. One of the reasons I wanted Evolution to work for me was the baked-in calendar functionality (as opposed to the extension/add-on method that Icedove/Thunderbird). The speed and stability of Evolution were respectively slower and less so than in Icedove, dampening my enthusiasm for the GNOME mail client. I'm not sure yet about Google Contact Sync. Last time I tried it, I had lots of screwed-up contacts (names and contact info mixed up).