I often use my apps on full-screen mode, since there are many distraction that I don't "often" use or need to click on (including taskbar, time, search-bar, icons, etc.)

However in Firefox I usually have several (if not more) tabs open, and I find myself needing to transit among them too. right now I have 8 + 6 pinned tab :) )
the default behavior is a smooth animation where you move the cursor to the top of the screen and the tabs(-bar?) collapses down. this is rather slow and requires an extra gesture or two for me, making it annoying to get around

To always show the tabs in full-screen mode, open about:config in Firefox, search for browser.fullscreen.autohide, and set it to false. there, now you can press F11 to enter full-screen mode while having your tabs visible.

credits to Happy112 from Mozilla forums | link