This is considered a "Legacy Add-On" so will not work with Firefox 57+!!! PLEASE UPDATE YOUR ADD-ON, MIGRATING IT TO THE NEW FIREFOX ADD-ON
Note: In Firefox version 57, in release on November 14, 2017, only extensions built with this new technology will work in Firefox. A majority of the most popular add-ons are already compatible, so most users should not notice any changes.
To see if you have incompatible extensions installed, click the menu button Fx57menu and choose Add-ons (or enter about:addons in a new tab) to open the Add-ons Manager. From there, you should be able to find recommended replacements for any legacy extensions. You can read more about this feature in this Add-ons blog post.
If you are the developer of a legacy add-on, RESOURCES ARE AVAILABLE TO HELP YOU MIGRATE TO THE NEW STANDARD:https://wiki.mozilla.org/Add-ons/developer/communication to help you migrate to the new standard.