Suddenly its gone. I really need to be able to zoom with a one-handed tap on the trackpad. Its homepage gives me no useful information, so I can’t even find the developer to see WTF is going on.
Somehow it went into the overflow menu. Weird.
Overflow can happen automatically depending on window width, other buttons on the bar, etc. Occasionally the address bar gets greedy and becomes unreasonably wide. Removing the xulstore.json file from the profile folder resets the width of the bar as well as window sizes and positions back to default. (It also will hide the menu bar and bookmarks toolbar if you turned those on.)