Despite following tumblrinaction for a few years I had never heard of this story. Here is the Xkit guy's story: http://xkit-extension.tumblr.com/post/69793258965/a-personal-message-from-the-xkit-guy
And here is the woeful attempt at picking up his work: http://new-xkit-extension.tumblr.com/
With this delicious tidbit right in the header:
>We've decided to update XKit. It's harder than it looks.
Install the new X-Kit. Aside from it being an awesome extension for Chrome and Firefox their newest version stops this from happening.
http://new-xkit-extension.tumblr.com
E: The problem is caused by staff botching an update that added video ads to the sidebar.
Another thing, there is an add-on for tumblr called "xkit" - it has a number of features that might be useful once you figure out tumblr's basics.
As a last bit, if you're looking for pony tutorials / guides for any reason.
Spambots are a good time and Tumblr is a functional website.
I would recommend getting New XKit or Tumblr Savior (I prefer the latter; New Xkit's blacklist seems to have fewer options) and blacklisting "goo.gl" to cut down on the spam.