The default screen sharing client for Ubuntu is called Vinagre (in later versions they have started calling it Remote Desktop Client).
On Crossbrowsertesting.com make sure you go into the Test Center, click on the Run live test button, and click the Live Test Settings link that is at the top of the page. On this screen, click the “use local VNC client” checkbox.
Start your session on Crossbrowsertesting.com and also start your VNC client. Type Alt+F2 then type in Vinagre (you can also run the Remote Desktop Client from the menu).
Once the address and port connection information pops up (it will look like livetest.crossbrowsertesting.com:60XX) copy and paste that connection string. Click on connect in the VNC client and paste the string in. Then type in your password and you should be connected.
When you are done remember to close the session on the console just like you would have if you were running the Java applet.