Issue since iceweasel was dropped in debian. same thing happens in firefox. Also can't use theatre mode on youtube videos. Browser becomes unresponsive and I have to close it.
As for the firefox / iceweasel isseus... Do these also happen in QubesDebian templates?
Then check Qubes about this. Up to the Qubes help. Whonix forum in that case is the wrong place.
I'm gonna keep posting every issue I have in whonix in here whether you think its appropriate or not till you ban me.