To use CardSpace as authentication factor in the Internet Your browser has to support it with selector component. Till this moment selector was only available for Internet Explorer, now we can have it also for Firefox users. This is important as key success for wide adoption will be possibility to use it on multiple platforms and clients. This plugin is step in good direction.
If we are talking about Firefox – my friend Witek Rugowski posted on his blog about annoyance in Firefox. This browser won't allow connection to sites on non-standard port if this port is one of well known services ports. No workaround at this moment.