Fixed in Firefox 59.0.2, Firefox ESR 52.7.3.
CVE-2018-5148: Use-after-free in compositor
A use-after-free vulnerability can occur in the compositor during certain graphics operations when a raw pointer is used instead of a reference counted one. This results in a potentially exploitable crash.
Since this was never stabilized, please stabilize www-client/firefox-52.8.0 to address this as well as newer security bugs.