GUIPropView is a tool for Windows that allows you to do many things (Hide, show, minimize, maximize, disable, enable, and more...) from command-line on the desired windows, according to their properties (Class Name, Title, Process, and so on...)
In order to minimize all opened windows of Chrome Web browser, simply run the following command:
GUIPropView.exe /Action Minimize Class:Chrome_WidgetWin_1 Process:chrome.exe Visible:Yes

The above command requests to find all visible windows (Visible:Yes) that their class name is Chrome_WidgetWin_1 (Class:Chrome_WidgetWin_1) and their process is chrome.exe (Process:chrome.exe) and then minimize all found Windows.

If you want to minimize all opened windows of Firefox Web browser, simply replace the class name and process name to the class name/process of Firefox:
GUIPropView.exe /Action Minimize Class:MozillaWindowClass Process:firefox.exe Visible:Yes

If you want to minimize all Windows of another Web browser or any other software, simply find the window of the software in the upper pane of GUIPropView, and then replace the class name and the process name in the above command samples to the class/process of the software: