On November 14th, 2017 Mozilla Firefox released a new version 57, which is also known as the "Quantum" release.
The browser now uses the new framework for its add-ons:
As a result, our SDL Clipboard Extension for Firefox (1.1.3) is no longer compatible with Firefox 57.
While copy-pasting is still possible without the extension, this add-on provides the best experience.
Therefore to avoid any unexpected behavior in pasting, we advise you not to upgrade to the latest Firefox.
Keep track of Xopus Blog to see when the new version of our add-on , that is compatible with Quantum, is available.