Perhaps related to Bug 910023 or Bug 1127029. Clicking select dropdowns with a tap works as expected. When actually pressing the touchpad button, the select is typically expanded and selected immediately. After selection the field remains selected (complete with blue ring). Clicking on a different element can expand the selected one! This is eminently reproducible on Bugzilla. Using a non-e10s window makes the problem go away. I'm uploading a video to YouTube now. This might require a Force Touch capable MBP to repro; I switched machines and upgraded to Yosemite at the same time, which was about the time I got bumped to e10s, so apologies for not having clearer prerequisites.

Video: https://www.youtube.com/watch?v=ZWt6wUShVLM

Turn the sound up to hear the difference between 'tap' and 'click', by the way.

thanks for filing with the video!

Since the list is expanding in the downward direction in the video, I actually suspect this is a dupe of bug 1152585.