Comment # 3 on bug 1198817 from Johannes Krottmayer

Sorry for my late response. Currently I renovate my apartment because of
technical issues with my home network. Today I will receive a big package with
CAT8 Ethernet equipment. :) Used too old cables for my network... But 40GBit
with CAT8 should be enough for the next 10 years. ;)

I have narrowed down the issue:

Firefox and Thunderbird requires currently a IPv6 connection. If this the
router from the ISP only supports IPv4, these Mozilla programs are very slow.
(Very slow startup and handling.

I think I don't need to provide a list of the installed packages. There are no
troubles with the packages.