With latest Firefox Nightly using wayland (GDK_BACKEND=wayland) on latest Sway, I'm having rendering issues. There is tearing windows are misaligned with containers and the background is leaking. Anybody else see this? Not sure if this is a Mozilla change or Sway change that started causing this. Also fullscreening a firefox window only renders on half the screen for me.
Here are three firefox windows with google opened in all three.

2019-10-19 21:08:41 - [sway/desktop/transaction.c:411] Transaction 0x559b0bb0f1d0 committing with 4 instructions
2019-10-19 21:08:41 - [sway/desktop/transaction.c:280] Applying transaction 0x559b0bb0f1d0
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0c0aeeb0 (res 0x559b0bf2b890)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0c108960 (res 0x559b0bbdb960)
2019-10-19 21:08:42 - [xwayland/xwm.c:830] XCB_CREATE_NOTIFY (4194306)
2019-10-19 21:08:42 - [sway/desktop/xwayland.c:590] New xwayland surface title='(null)' class='(null)'
2019-10-19 21:08:42 - [xwayland/xwm.c:844] XCB_DESTROY_NOTIFY (4194306)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0bb9e870 (res 0x559b0c2ec7d0)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0c0aeeb0 (res 0x559b0befee20)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0c108960 (res 0x559b0c2ec7d0)
2019-10-19 21:08:42 - [types/xdg_shell/wlr_xdg_surface.c:441] new xdg_surface 0x559b0bbbcbd0 (res 0x559b0bfb4540)
2019-10-19 21:08:42 - [sway/desktop/xdg_shell.c:497] New xdg_shell toplevel title='Firefox Nightly' app_id='firefox'
2019-10-19 21:08:42 - [types/wlr_server_decoration.c:106] new server_decoration 0x559b0c0e52c0 (res 0x559b0bee21d0)
2019-10-19 21:08:42 - [sway/tree/root.c:231] Looking up workspace for pid 41039
2019-10-19 21:08:42 - [sway/tree/view.c:343] View 0x559b0bba4eb0 updated CSD to 1
2019-10-19 21:08:42 - [sway/desktop/transaction.c:411] Transaction 0x559b0c111560 committing with 5 instructions
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0bb9e870 (res 0x559b0bbd2a70)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0c0e97e0 (res 0x559b0bf5cde0)
2019-10-19 21:08:42 - [types/wlr_surface.c:609] New wlr_surface 0x559b0bb99240 (res 0x559b0bfd12a0)
2019-10-19 21:08:42 - [sway/desktop/transaction.c:376] Transaction 0x559b0c111560 timed out (1 waiting)
2019-10-19 21:08:42 - [sway/desktop/transaction.c:280] Applying transaction 0x559b0c111560
2019-10-19 21:08:42 - [sway/tree/container.c:1133] Container 0x559b0bf78c40 entered output 0x559b0bbd9ae0
We have a workaround thanks to dano.kutka via https://bugzilla.mozilla.org/show_bug.cgi?id=1589114#c11:
Workaround is to switch gfx.webrender.force-disabled to true in about:config
Latest nightly fixed it for me, looks like it's been marked as resolved in the bug report on Firefox's end as well.
Most helpful comment
It's never sway.