The Classic Theme Restorer (Customize UI) for Firefox web browsers will restore the classic look and feel of Firefox 29+.

This add-on restores squared tabs, the application button with application menu (known from Firefox for Windows), the add-on bar, bookmarks/history menu/sidebar buttons and other buttons. It moves star & feed buttons into the location bar, adds status bar element to the UI and allows many items to be movable. All features are fully optional. Some buttons have to be moved to toolbars manually using browsers 'customizing' mode.

Classic Theme Restorer for Firefox Features:

Tab appearance: Squared classic tabs
Tab appearance: Curved tabs (all colored and visible)
Tabs not on top + [tabsontop="false/true"] attribute
Tabs height settings
Tabs min/max-width settings
Tab close on all tabs, active tab, toolbars start/end
Custom colors for tab backgrounds, text and text shadow
Application button on toolbars
Application button on titlebar [Windows only]
Own title for application button on titlebar [Windows only]
Predefined/custom application button colors
Small navigation toolbar buttons
Movable back-forward buttons
Star button in location bar
Rss-Feed button in location bar
Movable status-bar panel
And much more.....