Found the solution:
about:config
ui.use_unity_menubar=false
Who thought it was a good idea to enable it by default?
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https://bugs.launchpad.net/bugs/1663763
Title:
Firefox menu bar
Public bug reported:
On Xubuntu/desktop 16.10
- Menu bar appears on indicator
- Menu bar cannot be enabled in customize
This is a deal breaker. One of the main reasons why I use Firefox rather
than Chrome on Linux is the menu bar.
Should I use Chrome instead?
ProblemType: Bug
DistroRelease: