I am dealing with this too.
Seems the first 5 links on the first couple of google searches when Firefox is loaded, links to ad2.doubleclicker.net
Yesterday I noticed it was ad1 and not ad2.doubleclicker.net, odd that the url changed in a day...
I used all the malware and AV tools available, I even copied the entire firefox directory from a non infected computer and it still persisted.
In safe-mode firefox didn't seem to exhibit the behavior
When starting in firefox in safe mode you are presented with a dialog opting to disable all extensions, plugins etc. I selected them all and restarted, and voila its gone. (Even now when not in safe mode)
Now I tried doing this myself before resorting to the safe mode fix, disabling all extensions and add ons and nothing seemed to work.
Using DLL Dependency I compared with a non infected machine all DLL's that firefox.exe depends on and they all matched so I'm assuming its something in the plugable extensions that was the culprit.
So it's probably on my machine still, but just not loaded now by firefox..
What does safe-mode restore options actually do? Its too bad I can't dint he offending DLL/etc that was causing it because I know its still on my machine and no av or malware tool is detecting it.