What to KnowTo customize tools, go to the Hamburger Menu > Customize > drag-and-drop tools where you want them.To change the overall appearance, select Themes and choose one of the available themes.
This article explains how to add, remove, and rearrange the buttons on the toolbar and customize Mozilla Firefox's appearance for all operating systems.
How to Customize the Firefox Menus and Toolbars
To personalize the Firefox interface to your liking:
Select the hamburger menu in the upper-right corner of Firefox and choose Customize from the drop-down menu.
Drag-and-drop the available tools where you want them, whether that be on the toolbar or in the overflow menu.
Remove or rearrange buttons by dragging-and-dropping. You can also drag the browser's search bar to a new location if you wish.
At the bottom of the screen, select the Title Bar check box to display the web page title.
Select Toolbars, then select Menu Bar and Bookmarks Toolbar in the drop-down menu to reveal the respective toolbars.
Select Themes, then choose one of the available themes. Or, select Get More Themes for additional options.
Select Density, then choose the layout you desire.
Select Done to save your changes.
Select Restore Defaults to revert all customizations you made and return the Firefox interface to its original state.