The gist of it seems to be that its a set of data models that make handling concurrency control and conflict resolution easier in the context of a single blob of data being edited by multiple users simultaneously (similar to Google/Apache Wave)
Firepad doesn't actually do OT of JSON. In Firepad, a string OT algorithm is used to merge text edits, and the metadata describing rich text is separately managed as span objects, which get modified in accordance with text changes.
Firebase provides conflict resolution of JSON structures using their Transaction primitive, which allows them to create a Software Transactional Memory used to update the journal that powers the text OT in Firepad. ShareJS inside of Derby uses a similar approach to allow multiple servers to access the journal, but it is done in Redis with Lua scripting.