This really isn't the place to talk about pre-release builds of Firefox, but now that you ask, you need to make the Menu Bar visible for that feature to work. Go into View > Toolbars > Menu bar or Firefox > Customize > Menu bar. You'll see that it works.

When there's the Firefox button only, that double-click-open-new-tab doesn't work and it's not supposed to work. Unless you have tabs on bottom, in which case it's a bug, I believe.

jeap... I have the tabs on bottom, so then it is the bug, I think.

Yeah, I already filed the bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=577459