Like a few other Thunderbird users, I've found the Buttons! add-on to be extremely useful. Unfortunately, it hasn't been updated in a while, and it won't work with the latest update to Thunderbird. (It worked in 3.0.x, but not in 3.1.x)
I decided to try the easy way out to see if I could get it to work. I unzipped the Buttons 0.5.3.2 XPI file, modified the maximum compatible version in the install.rdf file, and rezipped it. Seems to be working fine for me. It's nice to have my Previous! and Next! buttons back again!
For those interested in using the version I tweaked, here's a link to the buttons 0.4.3.2b XPI. (yes, I added a "b" to the version number)