[Solution] Microsoft Lync meetings do not start with Firefox 21 or later

Symptoms

After upgrade to Firefox 21 or later, joining a Lync meeting with URL does not work if link is open in Firefox (when Firefox is default browser).

At the same time Lync link successfully opens meeting with Internet Explorer or Chrome.

Solution

Just copy npMeetingJoinPluginOC.dll from
%Program Files (x86)%\Mozilla Firefox\plugins
to
%Program Files (x86)%\Mozilla Firefox\browser\plugins
(create this directory if it does not exist yet).

FF restart is not needed, just open Lync link in a new tab.

Explanation

This behavior is caused by the new Firefox directory structure that was introduced in Firefox 21.

P.S. It seems that you need to repeat this operation after each Firefox update.