Using Firefox Nightly 88.0a1 (2021-03-05) (64-bit) on sway with MOZ_ENABLE_WAYLAND=1, popups (right-click popup, HTML drop-downs, FF main menu, bookmarks toolbar folders, etc.) flicker. The flicker occurs intermittently, most often when a popup is first opened, when the mouse moves, or when the popup covers dynamic content (e.g. video). The flicker can be significant enough to make menu items difficult to read and select. Attached video demonstrates the flickering. Mozregression:
Last good revision: 8a964fae4408a2710696df83c1e34388801f484c
First bad revision: 29a6032df4108887a29d11301c6151fe2a9b956c
Which suggests it was regressed by the fix for Bug 1644018. My apologies for not mentioning it earlier. I had noticed the flickering but only recently had the time to investigate and bisect.