I had the same problem after upgrading Thunderbird and your fix (below) worked for me.
1) While in Thunderbird (obviously), open one of the emails to read it.
2) Right-click on the Tool-bar and selected "Customize...".
3) Drag the broken "Back" and "Forward" buttons into the pop-up window that contains the selectable buttons. (Yes, you are removing the two buttons that do not work.)
4) Drag the "Previous" and "Next" buttons from the pop-up window and drop them on the tool-bar.
It is interesting that after making the above changes, if you go back into Customize and click the "restore default set" button, Thunderbird puts it back the way it was:roll: , with the greyed out "back" and "forward" buttons. That tells me the staff at Mozilla haven't quite gotten all there ducks in a row on a recent change to Thunderbird...
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6