Abbeyphone Labs has launched Firefox and Thunderbird VOIP extensions for Mac, Linux and Windows (was available earlier). (via 21talks)
Currently available as a Basic version, with the final release, three types of extensions will be available:

Basic: Works with an Abbeyphone account
Popular: Will work in conjunction with popular VoIP services and clients: Skype, Gizmo, Yahoo etc. So, you can easily use these extensions together with your favorite service
Custom: Use any SIP proxy/registrar you want like Asterisk

Download Links-

Firefox
Windows- http://labs.abbeyphone.com/firefox/abbeyphone-ff.xpi
Linux- http://labs.abbeyphone.com/firefox/abbeyphone-ff_linux.xpi
Mac- http://labs.abbeyphone.com/firefox/abbeyphone-ff_mac.xpi

Thunderbird
Windows http://labs.abbeyphone.com/thunderbird/abbeyphone-tb.xpi
Linux- http://labs.abbeyphone.com/thunderbird/abbeyphone-tb_linux.xpi
Mac- http://labs.abbeyphone.com/thunderbird/abbeyphone-tb_mac.xpi