Early january 2020 or late december 2019, Gmx made a modification that changed the behavior of the "open email" icon (see screenshot). Now it opens the emails within the gmx webmail interface instead of opening a popup window. The functionnality is still available, but I could not find an option in the interface to change it. So here is a userscript to do that.
The script adds a checkbox next to the buttons toolbar (see screenshot). When checked the popup behavior is enabled, when unchecked it's the current default. The script remembers your choice.
On the first installation, the option is automatically set to checked. If the userscript is installed outside of a script manager (Tampermonkey, Greasemonkey, Violentmonkey, ...), it uses an alternative method to store the checkbox state. It remembers the state as long as the window/tab is open.
The script has been tested on gmx.net, gmx.com, gmx.fr, web.de.