Rated 4 out of 5 stars
TB 31.2.0 + Tag Toolbar 0.8.35
When trying to change character encoding via Option - Encoding - something in message compose window, error popup appears and encoding cannot be changed.
ReferenceError: BrowserSetForcedCharacterSet is not defined
Disabling Tag Toolbar solves this problem.
With TB 30, no problem.
Similar problem seems to be caused also by Priority Switcher extension: https://addons.mozilla.org/en-US/thunderbird/addon/priority-switcher/reviews/603200/