If the yellow security bar shows up at the top of the window when you click the link, then click the button on the yellow bar that says "Edit Options...", then click the button on the window that pops up that says "Allow", then click "OK," then click the link again, and the install should start.
Once the install starts, wait for the "Install" button to become enabled, then click it. The Extensions window will pop up. After a few seconds, the entry for the Orb Communicator will say that Firefox needs to be restarted to use the extension. Close down all open Firefox windows--including the download window, any browser windows, and the extension window, among others, and then open Firefox again to restart it.
To use the extension once installed, go to the Tools menu and choose the Orb Communicator option.