We were hoping to avoid these for Thunderbird beta 1, however, we needed to add one string to Thunderbird, and are expecting to add several more strings in the next day or so.
The bug that Mark Banner (Standard8) checked in yesterday is:
Bug 455801 - Polish changes to message reader for b1
This adds one string into mail/locales/en-US/chrome/messenger/msgHdrViewOverlay.dtd called "showDetailsButton.label".
It is visible on the message reader pane, when you have collapsed the headers (using hide details).
We will ensure that details of the other string checkins are posted as soon as they happen (and they take place as soon as they can). If you want to take a look at them before they happen, they are bug 455812 and bug 455978.