New features and API:

• loading and loadingProgress properties on webpage
• onResourceError parameter contains now always status and statusText properties
• proxy() and setProxy() methods
• slimer.version and slimer.geckoVersion have a new prerelease property.
• With Firefox 56 and higher, SlimerJS can be trully headless by adding the --headless option on the command line
• Proxy config: support https URI for pac
• new fs.isSpecial()
• onResourceRequested: support of requestData.postData when http method is “patch”
• Compatibility with Firefox 59

See change details in release notes