In some browsers, the Authentic Browser plug-in can also be installed manually.
There is no way to manually install an Internet Explorer add-on contained in a CAB file. Use Manual Installation via MSI.
Drag and drop the XPI file into the Firefox window. The Add-on Manager (Tools | Add-ons | Extensions) should display the Authentic Browser plug-in in its list of installed extensions.
For information about versions, see Authentic Browser Versions