This userscript highlights the folders where new emails have arrived by using a red background on the message count. It keeps track of how much unread emails there is in each folder, and which one are marked, so that this info isn't lost when you close the window or logout.
A folder is unmarked when the unread count decreases (you've read an email). You can also unmark a folder - without the need to read a message - by pressing CTRL + left-clicking the folder's spanner next to the message count (see screenshot).
Upon installation of the script, you must reload the webmail page/tab. Folders will then be marked when new emails are received.
This is a work in progress. I can add an option interface to change the marked color (if you are already using userstyles to change the default colors, and red doesn't work well) as well as other settings (eg. never mark specific folders).
For more userscripts, styles, extensions, tips, ..., visit my programming page.
I have another userscript for gmx (standalone view) to add the possibility to open an email in its own stand-alone popup window (as it used to be).
If you're german or spanish, and find this script useful, you can propose translation of this text in your language. Any help is welcomed.