I have big sympathies for FireFish, but I feel like the target audiences so far are very different. Maybe call it the network effect, but it’s difficult for me to imagine a tumblr user who maybe is used to blogging about their SuperWhoLock fandoms moving to FireFish where there probably is no big audience for that. At the same time I would want to see these users and their niches in the Fediverse.
There is a very particular culture that is unique to tumblr which I don’t see moving somewhere else.