Assume that the default browser is Firefox and its version is greater than or equal to 52. When you try to click a Skype for Business meeting link to join the meeting that's created at an on-premises server, the Skype for Business web client is opened to join the meeting instead of the Skype for Business desktop client.
This issue occurs because the Netscape Plugin Application Programming Interface (NPAPI) plugin is deprecated in Firefox 52 or a later version. NPAPI is currently used as part of the client detection process for the Lync or Skype for Business client when you join a meeting by clicking a link. When NPAPI is disabled, the webpage can’t determine whether you have the client installed.