See testcase, when focusing the first option in the multi-select, you see a 1px scroll happening. This regressed between 2010-01-11 and 2010-01-13: http://hg.mozilla.org/mozilla-central/pushloghtml?startdate=2010-01-11+04%3A00%3A00&enddate=2010-01-13+09%3A00%3A00 Regression from bug 526394.

20160502172042 Mozilla/5.0 (Windows NT 6.1; rv:46.0) Gecko/20100101 Firefox/46.0 20160525063710 Mozilla/5.0 (Windows NT 6.1; rv:49.0) Gecko/20100101 Firefox/49.0 I have tested your issue on latest FF release 46.0.1 and latest Nightly build and could not reproduce it. Is this still reproducible on your end ? If yes, can you please retest this using latest FF release and latest Nightly build (https://nightly.mozilla.org/) and report back the results ? When doing this, please use a new clean Firefox profile, maybe even safe mode, to eliminate custom settings as a possible cause (https://goo.gl/PNe90E).

Considering the fact that I cannot reproduce this and the fact that the reporter did not answered to my request until now, I will mark this as Resolved-Worksforme. If anyone can still reproduce it, feel free to reopen the issue and provide more information.