I managed to update Firewolf so that it is able to be used with Minecraft 1.13. Mojang changed a lot of directory and file names so without this update 3D models were appearing as broken textures in the game. I would like to personally thank Mojang for implementing such great backwards compatibility (absolutely no hint of sarcasm).
Currently, connected textures and the randomised bookshelf textures are not working with OptiFine. Please reach out to me if you know what the issue is.
I think I know why connecting and randomised textures are not working. According to the 1.13 update changelog for OptiFine they have changed the folder structure. I will try to fix the textures soon and will push an update.