The problem + screenshots on the FreeBSD forums: https://forums.freebsd.org/threads/what-the-heck-happened-with-my-firefox.76730/
Clarification: this is a problem of Firefox on DragonflyBSD. On FreeBSD, it was suddenly gone by reinstall the system with XFCE instead of MATE but the same procedure doesn't work on DragonflyBSD. The problem still persists.
