I sent this over mail, but just in case it was not received:

Hi Mihailo,

My name is Asaf Romano. I'm a member of the Firefox team at Mozilla Corporation.

As part of an ongoing effort to switch the Places Bookmarks & History system to 
asynchronous APIs, we're slowly deprecating the annotations API.

The first step in this direction is the removal of "binary annotations". This 
change is about to land on mozilla-central repository in the next couple of 
days affecting nightly build initially. Firefox Beta will see this change in 
nine weeks, and the official Firefox release will see this change in about 
fifteen weeks.

I've learned that your addon, New Tab JumpStart, utilizes this API for storing 
thumbnails for its new-tab-page implementation, and therefore wanted to provide 
you with few alternative APIs.

First in order is PageThumbs.jsm, a Javascript Module which provides the very 
same functionality. I cannot guarantee you this module won't change over time 
because it is quite new. However, it uses long-standing APIs, so another option 
is to clone it in your addon (using a different storage directory in the 
profile).

Finally, should you still wish to use the annotations API, a possible solution 
is to store the image files in a sub-directory under the profile directory, and 
then point to these files in regular string annotations. I don't recommend this 
approach because, as I noted above, the annotations api is about to be 
deprecated and very likely removed at some point.

Should you have any question with regard to implementing any of these options, 
please let me know.

Regards,
Asaf