Symptoms
Open in browser option is missing for an additional domain or a subdomain in Plesk File Manager drop-down menu.
Cause
Plesk bug with ID PPPM-9885.
Resolution
Manually paste to the browser the domain or subdomain URL and append it with the path to the file and file name.
For example:
Domain name: sub.example.com.
File location: /sub.example.com/newfolder1/page.html (where /sub.example.com is the document root).
Resulting URL: sub.example.com/newfolder1/page.html.