If you visit a popular community like /c/memes@lemmy.ml with your web browser, the images shown are hotlinked from the Lemmy instance that the person posting the image utilized. This means that your browser makes a https request to that remote server, not your local instance, giving that server your IP address and web browser version string.
Assume that it is not difficult for someone to compile this data and build a profile of your browsing habits and patterns of image fetching - and is able to identify with high probability which comments and user account is being used on the remote instance (based on timestamp comparison).
For example, if you are a user on lemmy.ml browsing the local community memes, you see postings like these first two I see right now:
You can see that the 2nd one has a origin of pawb.social - and that thumbnail was loaded from a sever on that remote site:
https://pawb.social/pictrs/image/fc4389aa-bd4f-4406-bfd6-d97d41a3324e.webp?format=webp&thumbnail=256
Just browsing a list of memes you are giving out your IP address and browser string to dozens of Lemmy servers hosted by anonymous owner/operators.
This isn’t about a firewall. This is about some image hoster figuring out and keeping records of your usage times, location tracking, for your account that comments and posts.
Lemmy is at a point where typical new-user sign-up had no terms of service, privacy policy, cookie policy - people need to understand that this is not a hardened and mature platform. There could be a lot of things that people didn’t expect - that a major corporation like Reddit or Twitter would be scrutinized more closely over.