Can I hide the Firefox title bar AND still see the close/minimize/maximize icons?

Firefox version 60 comes with an inbuilt client-side decoration (CSD) option which gets rid of the title bar but keeps the window control buttons (minimise, maximise, close).

If you have Firefox 60, go to "Customise" screen and disable "Title Bar" option at the bottom.

Then you should see something like this:

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As a side note, the pointy white corners that appear after turning off the 'Title Bar' option, can be removed by the following steps:

1. Open about:config in a new Firefox tab.
2. Accept the risk and continue.
3. Type mozilla.widget.use-argb-visuals in the search field.
4. Check that 'Boolean' is selected.
5. Click the '+' button to add the preference setting.
6. Restart Firefox.

Source: article on OMG! Ubuntu

Try this, it solved my problem. This is how Firefox looks after I installed it:

The close/minimize/maximize buttons are near (on the right of) the power icon (top right of the screen, they only show when you move the cursor there But you can modify that on Tweak Tools >Extensions >No Title Bar after you install it).

I use the GNOME Shell extension Pixel Saver for this.

When a window is maximized, this extension removes its title bar and places the minimize, maximize and close buttons in the top bar.