Dear Maintainer, Filed upstream at https://bugzilla.mozilla.org/show_bug.cgi?id=1344620 User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:51.0) Gecko/20100101 Firefox/51.0 Build ID: 20170125094131 Steps to reproduce: I'm on Firefox 51 on Debian Jessie. This is reproducible both on upstream firefox (from getfirefox.com) and on the Debian-packaged one (still 51, from below sources.list.d): --- deb http://mozilla.debian.net/ jessie-backports firefox-release deb-src http://mozilla.debian.net/ jessie-backports firefox-release --- 1. Create a fresh profile (or even a fresh OS user). 2. Go to example.com (or any other site) 3. Try to open right-click menus, the menus at the top, and the hamburger menu Actual results: Often these menus (right-click menus, the menus at the top, and the hamburger menu) will not open. It then sometimes resumes working. Expected results: The menus should open normally each time.