I forgot: My current (old) setup or my connection to the WWW: ISP Router (VDSL 300MBit downstream) -> Linksys WRT32X (with OpenWrt) -> To the devices (PC, SMart TV, PS4 Pro). When I disable IPv6 in my main router (LinkSys WRT33X) Firefox and Thunderbird are extremely slow. Tried to disable IPv6 in about:config. But doesn't help.