+++ This bug was initially created as a clone of Bug #1128159 +++ We allow dropdown widths to exceed the anchoring element if it is necessary to display the text in the dropdown. This is because the XUL menupopup has pretty large padding. This bug is not particularly visible on Windows because menupopups don't have much padding there, and so we mostly match the width properly.

I'm not convinced this is M8-worthy. Kicking out to triage.

this want for release if we can fit it in.

er - "this is a want to fix before release if we can fit it in"

Now that we have M9 and M10, I guess based on comment 3 this maybe could fit into one of those.

Hi, I couldn't reproduce this issue on the latest Aurora (46.0a2) using the testcase from Bug #1128159. I've tried to reproduce this on Windows 10, Windows 7, Ubuntu 14.04, Mac OS X 10.6.8 and 10.10.5 and in all cases the width of the dropdown menu properly fits the anchoring element. Build ID 20160204004009 User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:46.0) Gecko/20100101 Firefox/46.0 User Agent Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:46.0) Gecko/20100101 Firefox/46.0 User Agent Mozilla/5.0 (X11; Linux x86_64; rv:46.0) Gecko/20100101 Firefox/46.0 User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:46.0) Gecko/20100101 Firefox/46.0 Considering this I will mark this issue as Resolved-WFM. Thanks, Cipri