If you are using a very old version of sway, you might have noticed that in the latest Firefox update tooltips and other windows are uncontrollably flickering. This is an issue that has been reported a couple of days ago already¹²³,and recently the fix has been merged to sway master⁴.

To fix it you simply have to update your sway to master or to version 1.6-rc2.

cd $WLROOTS_SOURCE
git fetch origin
git merge origin master --ff-only
ninja -C build
ninja -C build install

cd $SWAY_SOURCE
git fetch origin
git merge origin master --ff-only
ninja -C build
ninja -C build install