From Wengowiki

How to start the plugin within FireFox ?

Once you have downloaded the extension from the Wengo web site, you need to add it to your Firefox plugins list and restart Firefox. For further details on how to add a plugin in Firefox, please refer to the WengoPhone for Firefox user guide.

The keyboard shortcut to start WengoPhone in Firefox is ctrl-shift-O. You can also start it from your Firefox toolbar.

Can I do video calls with WengoPhone for Firefox ?

No so far, video feature is not available in WengoPhone for Firefox.