Please read about it here: https://lab.xceptance.de/releases/xlt/latest/release-notes/4.10.x.html#toc-script-developer
As you may have heard already, Firefox 57 does no longer support XUL/XPCOM-based browser extensions (a.k.a. legacy extensions) in favor of extensions built upon the WebExtension API. As a consequence, Script Developer won’t install or work in Firefox 57 and above. Unfortunately, porting Script Developer to the WebExtension API cannot recreate the same user experience and functionality. Furthermore, user feedback has shown that the possibilities of XML script test cases are often too limited for today’s test automation needs. Therefore, Xceptance recently decided to discontinue the development of Script Developer . See our announcement for how we have come to this decision and which possibilities you have now.
Script developer runs on Firefox 45/ESR up to Firefox 56. We recommend using Firefox 52/ESR. Firefox 57 and later versions are not and will never be supported.