This bug is not currently tracked.
I see many reports through the contribute form of the "+" button not working. I'll see if I can get more details.
Could be https://support.mozilla.org/en-US/questions/1005683?esab=a&s=new+tab&r=4&as=s
All of these that I've triaged in fx::untriaged were caused by add-ons.
Ok, then let's WONTFIX.