I figured it out.
First of all, note Firefox allows you via right-click context menu (on an empty bar area) to enable
menu bar. This is not the same as
Via right-click context menu > Customize … you can enable
title bar. Enable that, and I have what I want:
Why did I start this topic?
Well I use vertical tabs, and always had a single line in my firefox profile/chrome/userChrome.css file to hide the default tabs bar. To still be able to close/minimize/maximize my Firefox, I had the Title bar.
(it seems pointless to have just those buttons in a Pixelsaver Panel. And in the rare case I need the
Alt menus, I can easily enable them temporarily via context menu).
Then something happened:
For some reason, I created a new profile, forgot to backup my .css file, googled what I had put in there and… put in the wrong code: hiding the contents of the tabs toolbar, instead of the tabs toolbar itself.
This is where the fun starts: I now had exactly the same bar at top that a fresh new profile has: the tabs toolbar with window buttons! I just did not recognize it as such, thought it was a title or menu bar (with hidden menus). In reality, it was an empty (no buttons for new tabs, no tabs) tabs toolbar.
As I was missing the default Budgie menu bar, I thought something was missing.
To fix it, I changed my userChrome.css to remove the tabs toolbar. That’s all!
To get my Window buttons back, I simply enable
title bar via context menu > Customize…
As an added benefit, Firefox looks like every window now, with the generic title bar.
Conclusion for others:
If you want Firefox top bar to look like any other Window, just enable Title bar. Note that title bar never contains the menus, those are in Menu bar.
During my testing, I also tried Pixel Saver. Had some weird behaviour like in the screenshot from @Coeur-Noir, Pixelsaver weirdly added the titlebar to the window during max/minimise. Not THAT weird, the weird par are the window buttons. But that makes sense: by default they are in Tabs bar. If Menu bar is enabled, Firefox moves them up, otherwise it would be very weird to have menus above your window buttons.
Hope this rant is useful for someone