I use zabbix4.0.1 in CentOS7(7.5.1804).
firefox is firefox-60.3.0-1.el7.centos.x86_64.

I upgraded it in 4.0.1 from 3.0.22 in 11/9.
I installed firefox in the machine which zabbix is installed .
I always display  screen  with graphs, but  Firefox uses memory so much like a attachment .

When  exiting FireFox, Memory usage recover.  
When I　used 3.0.22 , there was  not abnormal memory consumption
In addition, it was similar in Chrome.