The extension has been removed from Firefox's Add-On store (AMO) due to:
Acceptable Use, specifically Sexual content: This content contains sexual or pornographic content that violates Mozilla’s Acceptable Use Policy. (See https://www.mozilla.org/about/legal/acceptable-use/.)
Therefore, the Firefox extension's id has changed in v.0.0.8. This means that if you are updating your Firefox extension to version v0.0.8, you'll need to remove your existing "OF-DL Auth Helper" extension first to prevent duplicate extensions being installed.