We're in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
Zotero Preview pane (cslpreview.xul)
The Zotero Preview pane is a tool to preview CSL styles. It can be opened by entering chrome://zotero/content/tools/cslpreview.xul in the Firefox address bar.
The preview pane shows rendered citations and bibliographic entries for all installed CSL styles, using items selected in your local copy of Zotero.