I have spent many hours trying to resolve the very slow page loading in Firefox due to Doubleclick. The answer was there all along. Simply install No Script in your addons and disallow Doubleclick on those pages with this problem. Once this is done, the page loads much faster. I received a email today asking for more information and realized how short of info this post is. My reply to him follows.
Sorry I was so unclear, I was in a rush to get this out as I was leaving
I have been having a lot of trouble with very slow or no page loading in
Firefox on many websites as well as in Opera on my Ubuntu computers and
also on my wifes Win Xp setup including Explorer. In researching the
issue I found that several people had discovered that the issue was
connected to googleads and adsense which is somehow connected to
Doubleclick.. I began to suspect that Doubleclick was the culprit as it
showed up in most posts as a suspected culprit. So I decided to try to
control doubleclick using the NoScript extension. I installed NoScript
which allows me to control Doubleclick and any other problematic or
unwanted script. It allows you to be selective on problem web pages so
you are able to find the problem script. For me, as soon as I started
not allowing Doubleclick on problem sites they then loaded like
lightening . I hope this helps explain better what I am trying to say..
I'm no computer guru, just a good detective with a second sense.