Sure. I don't know anyone on firefish, but this is dansup of pixlfed on that platform: https://kbin.social/u/@dansup@pixelfed.social/posts
If you want to see posts from somebody on another platform just search them, something like "@ dansup@ pixelfed.social" (without the spaces) and follow them. From then on you should be able to see their posts (note you won't be able to see old posts, just any new ones from when you followed them onwards).