If the extension isn't visible in Firefox toolbar after you install it, do the following: 

Update (12/08/2015): with latest extesion version (1.0.2) the issue has been fixed, therefore this fix is no longer needed. Remove it and install the new one from https://vpn.ac/download/secureproxy.xpi

First check in Firefox menu > Customize if you can find the extension. If it's there, right click on it and send it to toolbar. 

If the above doesn't help, check in privacy settings if you have the browsing history set to "Never remember history". You need to change it either to "Remember history" or "Use custom settings" - this will still allow you to disable history, though do not check "Always use private browser mode" but clear history when exiting Firefox.