When using Firefox, lowering mtu to 400 causes problems.
If you lower mtu to 400 when using Firefox The problem occurs in the latest version 78.3esr.
The site doesn't open properly You can reconnect countless times over and over again, The page does not open.
This issue did not occur in the 68.12esr version.
There was no problem in the lower version even in the non-esr version. When the version was upgraded, the symptoms appeared. I've been using 68.12esr.
I can not speak English well.
Korea is a country that blocks sites using https. Adjusting mtu is a necessary measure for Koreans.