I have a solution (of sorts) for this.
First, remove the plugin.
Next, open the firefox advanced config by typing "about:config" in the browser.
Scroll down to xpinstall.signatures.required, and toggle it to "false" by double-clicking it.
Now, download and install the plugin again. It will be active and work.
Also, if you already have the above configuration set, you still need to remove the plugin, then just re-download it and it will work again.