GrabMyBooks Desktop release
As you had to find the hard way GrabMyBooks as an extension is not available anymore neither for Firefox nor for Chrome.
But here comes GrabMyBooks Desktop, a standalone version which does not require any browser to work.
Be sure to have at least one of the latest version of java 8 installed on your system.
New versions of the browser extensions are under development and should be soon available. They will allow to control GrabMyBooks Desktop directly from the browser.