1.Install Gnome tweak tool
2.Install Gnome Shell Integration Addon in your browser

We can Remove the Title Bar of an Application when maximized
For this purpose we can install the Unite Extension

Or

We can Hide the Top Bar of Gnome DE for maximized application
For this purpose we can install Hide Top Bar Extension

Only use any one of the extension.

Please Pause the video to view my extensions settings.

Links:

GNOME Shell integration (Firefox): https://addons.mozilla.org/en-US/firefox/addon/gnome-shell-integration/

Unite Extension: https://extensions.gnome.org/extension/1287/unite/

Hide Top Bar: https://extensions.gnome.org/extension/545/hide-top-bar/