I implement this in chrome, and this method should work in firefox.
Firstly, install an ad blocker extenstion like AdBlocker.
Secondly, open extenstion options interface and add following code snippet to your own filter:
youtube.com###movie_player > div.ytp-gradient-top
youtube.com###movie_player > div.ytp-chrome-top
youtube.com###movie_player > div.ytp-gradient-bottom
Then it wiil work.
An image descrip this:

How to find that filter, it also is easy, open your target website and click F12 key, do this first :

Then construct a formula defined by AdBlocker:
(Website's Host)##(The selector)
and here is:
youtube.com###movie_player > div.ytp-chrome-bottom
It should be noted that you need set the video full screen to find the selector when you want to block the title in full screen mode.
(There may be a grammatical error, but I don’t think it will affect your understanding.)