If you click the online meeting URL on a machine that already has Lync 2010 client installed, it will launch the Lync client as default. I tested it both in FireFox and IE, and they are all the same. If you want to force it to use Lync Web App, what you need to is to append ?SL= to the meeting URL.
This article explains how the process works: http://blogs.technet.com/b/jenstr/archive/2010/11/30/launching-lync-web-app.aspx
Jackie Chen's IT Workshop 