Arch Linux

Description:
Since a few days firefox (59.0.2-2) crashes when leaving its window unnattended for a random period of time (normally minutes). This is not reproducible, sometimes it doesn't happen.
After that there's no firefox process to track but its window remains open showing a black background with a white clock in the middle.
I've tried it with hardware accel set to off but it happens the same. I use i3-wm (4.15-1) as my window manager but this has also happened under gnome-shell.

Additional info:
There's no trace of firefox crash under dmesg output :(
After crashing I can close the window and restart firefox normally, it doens't say anything about having crashed before.

Steps to reproduce:
Start firefox, leave its window unnattended for several minutes, no matter which page it is showing. Sometimes this error happens, sometimes not.
When it happens I've noticed that I am usually working in another workspace, it never happened to me being working in the same workspace as the firefox window was.