I'm not sure if it's related, but I remember seeing something similar a few months back -- regarding the Pale Moon browser:
I know the title talks about the DWM window manager, but in the end the problem was resolved by downgrading the Mesa graphics libraries. Periodically a new graphics driver comes along for Intel integrated graphics chipsets (ahh, let me recall all of them over the years --
crocus -- I might be missing a few!). When a new driver comes out, Ubuntu (and everybody else into distributing these drivers) loves to jump on the new drivers immediately; stupidly, even the Ubuntu LTS releases are not always conservative about this. And the driver usually is buggy when it's first released. So random crashes -- sometimes crashes that bring down the whole GUI -- develop.
I honestly don't know how to safely downgrade Mesa, and furthermore you might be using some other graphics driver -- God forbid Nvidia, good luck with those. I'm just hoping somebody else here knows what to do with the information I've provided.