Release 5.4.0

Thanks to all contributors!

Danny Colin (1):
      Refactor install task in Makefile

Jesus (3):
      css: fade out url bar input overflow, improve spacing
      urlbar: show forward button if can't go back
      urlbar: fix missing suggestions

Oliver Smith (1):
      UserAgentManager: update user agents

Peter Mack (15):
      TabCounter.sys.mjs: observer-based refactor
      tabmenu: remove tiny grey divider on the left
      aboutmobile: Add sections for horizontal/vertical tabs and more
      boot.sys.mjs: Turn off Picture-in-Picture by default
      urlbar: don't double hide picture-in-picture with CSS
      preferences, sidebar: reduce empty space at the top
      urlbar: fix suggestions width with vertical tabs
      release/main.css: link urlbar.css
      boot.sys.mjs: Return to old pre-Nova about:home
      docs: document browser.newtabpage.activity-stream.nova.enabled preference
      about:mobile: Add a link to our docs page
      boot.sys.mjs: add extensions.unifiedExtensions.button.always_visible
      about:mobile: Move Firefox VPN preference to Unsupported Features
      about:mobile: Change wording for Telemetry and Studies toggles
      boot.sys.mjs: app.shield.optoutstudies.enabled -> false