Abbeyphone Firefox VoIP (SIP) extension
Abbeyphone is VoIP service provided by Abbeynet, but Abbeyphone Firefox VoIP (SIP ) extension developed by AbbeyLab only enables users make calls with Abbeyphone service via Firefox, but also support any standard SIP account.
A lot of VoIP providers developed their Firefox extensions, such as Skype, Jajah, Wengo and so on. But Firefox extensions like what developed by Skype and Jajah only are used to recognize digits on web pages that look like phone numbers, they only provide a convenient way to make calls. Firefox extension developed by Wengo is a complete VoIP client, but it doesn’t support VoIP providers other than Wengo. Abbeyphone Firefox extension is an open SIP client. Except pre-defined VoIP provides including Abbeyphone, FWD, Gizmo, VoIPCheap, VoIPStunt, users can add their own favorite VoIP providers based on SIP.
First install extension from MozillaVoIP.com according to your operations system — there are Windows、Linux and Mac OS to choose.
After installation, restart Firefox, there will be a windows to set SIP account.
Then you can make VoIP calls via tool bar at status bar or navigation bar.
This extension will highlight the digits on web pages that look like phone numbers. When cursor is pointed to those digits, there will pops up option to make call. Or when you choose a digits or strings and click the right button, there will be a new option to make calls on the menu.
Abbeyphone also has Thunderbird extension. Both of these extensions have been out for a while. I tested Abbeyphone Firefox VoIP extension and the quality seems to be OK, there is no problem to make or receive calls. There is also option of video, but it seems that this option only works with Abbeyphone account. People doubt about the target market of this kind extension, I guess it will be attractive to users want to do all jobs with Firefox anyhow 
Translated from Abbeyphone Firefox SIP网络电话插件
it seems that the addon is not FF 3.0 compatible any news about
when it will be ready