Dolphin and Konqueror are the main file system browser on the kde desktop on linux. And they can open remote ftp file systems with the "ftp://" urls. By default they use passive mode ftp, but can be configured to use active mode ftp as well.
The ftp protocol operates in 2 modes. Namely passive and active. Unlike http, ftp connection relies on 2 connections from 2 different ports. Now how the 2 connections are made, determine whether its an active mode ftp or passive mode ftp.
If the client make the first connection to the server and the server makes the second connection to the client then its active mode, since the server has actively connected back to the client. In passive mode however the client makes both the connections to the server, because the server is not able to connect to the client. This can be for many reasons. For example the client might be behind a firewall or router that does not allow incoming connections. To solve this, the passive mode ftp is used.
The configuration setting for dolphin to use active mode ftp is hidden in the KDE system settings. Go to
KDE System Settings > Network Settings > Connection Preferences. Over there you see the FTP Options. Uncheck the "Enable passive mode (PASV)".
Now dolphin and konqueror shall use active mode connection to ftp servers.
When ftp servers are installed on servers, then by default they are not configured to use passive mode. In such a case if a client connects in passive mode, then it will be able to login, but the directory contents would not be navigable and neither would any file upload or download work. Ftp servers need additional configuration to serve passive mode connections properly.
For a detailed explanation on active and passive mode ftp check out this article.