User Agent: Mozilla/5.0 (Windows NT 6.1; rv:40.0) Gecko/20100101 Firefox/40.0 Build ID: 20150521004003 Steps to reproduce: Originally encountered this in my Addons Manager Hilite add-on which uses a custom autocomplete listbox. But it also occurs in Thunderbird: 1. Add "address1@example.com" and "address2@example.com" to addressbook. 2. Click "Write", write "addr" in "To" field, autocomplete should replace it to "address1@example.com" and show a popup to choose. 3. Move mouse over autocomplete popup entry address1@example.com. 4. Move mouse over autocomplete popup entry address2@example.com. 5. Move mouse down completely out of autocomplete popup . 6. Now click outside of the autocomplete popup. Actual results: The item that was last highlighted before the mouse exited the popup is confirmed just as if it was clicked on. Expected results: The original item in the To: field should stay there and not be replaced by some item that was highlighted but never actually chosen.

Tested in Thunderbird 31.7.0; issue in my Fx extension occurs on Trunk (not sure how long previously). Possibly dup or related to Bug 1106778 ?

Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336