If you would like to use the ReadCube Bookmarklet with Firefox, please be aware of the following:
When you use the bookmarklet in Firefox for the first time, you may be presented with a message saying “This link needs to be opened with an application.” You will be prompted to choose an application to open the link.
When shown this prompt, simply choose ReadCube to open the link (in your applications on Mac, or in Program Files on Windows). If there is checkbox to remember this choice, check it.
Then, the ReadCube Bookmarklet should work seamlessly from now on!