Note: Starting with FreeCAD 0.17 persistent toolbars is part of the default FreeCAD experience.
PersistentToolbars can be installed as FreeCAD module. Install path for FreeCAD modules depends on the operating system used.
Linux:
/home/user/.FreeCAD/Mod/PersistentToolbars/InitGui.py
macOS:
/Users/user_name/Library/Preferences/FreeCAD/Mod/PersistentToolbars/InitGui.py
Windows:
C:\Users\user_name\AppData\Roaming\FreeCAD\Mod\PersistentToolbars\InitGui.py
Once installed no additional configuration is needed. Toolbar position changes made in different workbenches should be restored when FreeCAD is restarted.