The short answer is: this is the logo Mozilla sent us in their package of materials for their tag sponsorships (note this logo is also used on firefox-os and html5-apps).
To get to the bottom of where this logo comes from, just take a look at the logo for the Mozilla Foundation:
If you still don't see it, black out the dino head...
...and from there, I hope you can see where this logo comes from:
I can't speak to exactly why the logo above is being used, but I can assure you it was sent directly from Mozilla to the Ad Sales team here at SE.
Regarding the use of cdn.sstatic.net versus imgur:
We used to use the former for hosting those logos for tag spsonsorships, now we use the latter. Note that the examples you cite, android and facebook are sponsorships that have been around for a long time. Take a look at a more recent (though not that recent) example in azure: https://i.stack.imgur.com/SeoON.jpg
I hope that clears everything up! :)
In the comment thread of an article posted on Hacker News, Robert Nyman (Technical Evangelist for Mozilla) said the following:
Yes, I do. :-) That logo is the new logo for MDN, our network for developers at Mozilla. Since Stack Overflow is a resource for developers we see a direct connection to that. Also, we have different foxes for different products, so that would probably be confusing - rather have one consistent look.