Forked from @boobslider100@lem.ee's script - I just modified it to navigate Lemmy with arrows instead. I find it much easier than using J/K. Right/Left arrows have dual usage also, as they can both change the page and jump to next/previous same level comment :)

Down = Next post / comment
Up = Previous post / comment
Right = Next page / parent comment
Left = Previous page / parent comment
A = Upvote
Z = Downvote
X = Expand
C = View Comments
Enter = View Link

I made it for me but feel free to give it a go and let me know what you think!