I want to change how the title bar look and also the font with which Firefox is rendering the pages, but any change i made in fonts or windows decoration in system settings doesn't seem to affect Firefox. Why is that?

Browsers like to provide their own integrated title/header bars. You can change that in Firefox's Customize page by checking Title Bar in the bottom corner.
Browsers also provide their own default font settings. For Firefox this is under Preferences -> General -> Language and Appearance.

Yes, but how to change the min/max/close buttons, or tittle bar size? Because whatever window decoration theme i choose Firefox stays with the default Breeze theme.

You did enable the Title bar option in FF's menu -> Customize, correct? If that option is unchecked then FF will provide its own integrated (space-saving) title bar, using the styling from the GTK theme. When checked it should use the system decorations for the title bar. If that isn't working you may need to restart some things (Firefox and/or Plasma).