Mystery Firefox Download Sites

I use Gmail pretty much exclusively, and whenever I get e-mail having to do with Firefox or Mozilla, I usually see AdSense ads pointing to a host of strange "Get Firefox!" sites.

I've started clicking on them to see what's up, and I'm finding that, more often than not, they do nothing. Here's an example: http://downloadfirefox.ws/. Here's another.

So, what's the story here? These AdSense ads aren't free, so the operators must be doing something with them. I haven't spent really any time analyzing the landing sites, but I do notice that the links to download generally don't work, thus, I'm ruling out that they are actually interested in getting me to download FF.

Are these malware distribution sites? Spyware? Banner farms? Neither of the example sites are doing anything obvious like that (unless they're being unusually secretive and returning me different content based on my user-agent settings).

If you know, or have a theory, please comment below.