This has been occurring on one of my machines for the better part of the year. If I run firefox, the entire machine will lock up at some point. It could take minutes or it could take hours, but it will lock to the point of needing to hold the power button to shut down.
It seems more common when videos are being run, which makes me feel like it may be video driver related (It's an Ati graphics card).
Other browsers work just fine.
I've tried multiple different flavors of Linux, and it only happens in Manjaro. I've also tried multiple kernel versions.