Despite the claim made by the author, this is not a WebExtension. I can't understand why the author would make the claim in the first place; installing any version hosted by AMO will cause Firefox to tell you that a restart is required for the extension to work.
Also, no uploads have taken place since 2014, before the Firefox WebExtensions API was available.
And, finally, there is no contact information for the developer; no addon homepage; no bug report form; and no other way to communicate with the developer to ask her/him what's going on.
I wouldn't consider installing this extension until:
a) The developer lists a support site or other support / contact information, and;
b) Either retracts the WebExtension claim or provides a completely rewritten version of the addon that actually supports the claim made.
I use this constantly. Be it a web dev or netadmin, Firefox aggressively caches and can make you crazy as you develop content or have a lot of DNS changes. This makes it so easy to just get things working again.