User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0 Build ID: 20141011015303 Steps to reproduce: [Please see the attached image as it will be easier to understand the problem] [1] Compose a new message, write part of the TO address and let Thunderbird to auto-complete it while a drop-down list appears suggesting more addresses. [2] Move the cursor down to write a SUBJECT and you probably will hover one of the addresses displayed on the drop-down list. [3] The address highlights, but as you have not clicked it, the TO field keeps unchanged. Actual results: Be sure to not click the drop-down list. Click the SUBJECT field. The address in the TO field automatically changes to the highlighted address in [3] step. Expected results: As you have not clicked that address (just highlighted), the TO field should have remained unchanged.