Comment 8 for bug 2033988

EVEN BETTER WORK-AROUND:
It seems that the Firefox Snap team made a change to the shortcut that Firefox uses and it's the environment they've chosen that's causing the issue. They seem to be assuming that everyone is using Unity. Since this is a KDE machine, that environment doesn't work.

This is the default command inside the Firefox shortcut:
env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/firefox_firefox.desktop /snap/bin/firefox %u

If I replace that command with this, the issue mostly goes away:
env GTK_THEME=Breeze /snap/bin/firefox %u

That only works if you use the shortcut that contains that command to launch Firefox. If you launch Firefox by clicking a link in email or anywhere else, for example, the original issue is back. Also, although this mostly works, something causes it to occasionally happen while Firefox is open. When that happens, closing and restarting Firefox "fixes" it again.

This is not a perfect work-around, but it's better than the one above and at least works most of the time.

Note: If you use a desktop theme other than Breeze, you would replace Breeze in that last line with your theme's name.