I've been hitting exactly the same problem on Debian 8 running Firefox 49.0-5~bpo80+1 from https://mozilla.debian.net/. The issue seems to be solved in Firefox 50.0~b11-1~bpo80+1.
I've been hitting exactly the same problem on Debian 8 running Firefox 49.0-5~bpo80+1 from https:/ /mozilla. debian. net/. The issue seems to be solved in Firefox 50.0~b11-1~bpo80+1.