Firefox crashes computer only on hlwm

[famubu at rawtext.club]

Hi. I had been experiencing something strange over the past few weeks.

If I got firefox running, after some time the computer will get totally
stuck. Sometimes it happens within 5 minutes and sometimes it takes
around 30 minutes, but it *always* happens while on hlwm.

Tried updating firefox version (now at 96.0 on Ubuntu 18.04) and
clearing cache, cookie, etc but to no avail.

Interestingly, if am I am on xfce, this problem never shows up. I tried
switching to xfce and have been on it for the past 4 days with firefox
running, but there were no problems.

It seems that the problem happens only with hlwm.

Is it just me or have anyone had had some similar problem before?

Does anyone has any idea as to the reason behind this anomaly?