Hi !
I'm using Firefox Developer edition on sway, and since version 121 it shows a small border on top. I have the option hide_edge_borders both enabled, so this border is displayed by Firefox, not Sway (cf. the terminal on the right end of the screenshot).
Does someone else have this issue? Do you have an idea for a workaround? Is this a bug I should report to Firefox?
PS: I've seen this issue on Arch, NixOS and Firefox 122