By default, every Firefox launch, a new empty window will appear. But if you are accustomed to opening dozens of tabs, be aware that it is possible to restore them every time you start.
The procedure described here for MacOS X is very similar for Windows.
To do this, click on Firefox then Preferences.
On the tab General, in the part Start-up, click on the drop-down list At the start of Firefox and opt for Display the last tabs and windows used.
Close the window to validate, changes are immediate.
An indispensable option if you open many tabs in Firefox like me!