The bug does not exist on Windows. It does exist on a fresh profile in SeaMonkey on Fedora 22. Can you check for me whether it works properly in Firefox with a fresh profile?
bugzilla link: https://bugzilla.redhat.com/show_bug.cgi?id=1261182
It got auto-pushed to stable, and I'm not going to bother trying to replace it. Because it at least does its job now. I don't think it a good idea to try to replace a partially-functioning build with a non-functioning one while working on a fully-functioning build.
Regarding your bug, I can reproduce the behavior. However, I am able to fix it via about:config.
... You're right. I wonder what the problem is. The build process on our end hasn't changed in the least: unzip, get a directory listing, shove everything in the list into place. I wonder if it's the new AMO signature. It shouldn't be, but...