Thank god i found out this thread :)
I was thinking, that something is wrong with my custom pfSense box, but i can confirm, that this issues still resides.
If I started a download in Firefox and then used the same Firefox browser to view the dashboard for the traffic graphs - the download would slow down a lot. When i view the dashboard from Chrome, no problem with speed.
using: 2.4.4-RELEASE-p1 (amd64)
built on Mon Nov 26 11:40:26 EST 2018
FreeBSD 11.2-RELEASE-p4
EDIT: with the latest Firefox 64.0 version, the issue is gone!