Whenever i use Spybot after it detects Spyware and Adware, then after its fixed i run the scan again and it detects spyware and adware again!!! This happens everytime i rescan.
O17 - HKLM\System\CCS\Services\Tcpip\..\{6692504B-1B16-4C2C-9509-D936EAA6F916}: NameServer = 83.146.21.5 212.158.248.6<Only fix this, if it doesn`t belong to your ISP.
They`re just tracking cookies and nothing to worry about.
You can actually disable any website you choose from setting cookies in Firefox. See HERE for more info. Just scroll down the page untill you find the section on cookies.
To answer you first question. I`d personally recommend blocking those tracking cookies in Firefox.
As for your second question. A clean HJT log is a good indicator that your system is clean. However, it is possible for some viruses to hide on your system and not show up in HJT.
Unless you`re having some unexplained problems, I wouldn`t worry about it too much though.