I wanted to do that, and I started on an extension for it, but as others have said, lemmy being open source means that improvements should mostly be made on the website itself, rather than on a browser addon.

The downside is that for user settings, they are stored on the instance’s database, which means that you need to know a little bit of rust to implement a new setting (I’m doing that right now to open comments in a new tab instead of having to middle/ctrl click)