Oh my lovely firefox… Today I had to restart my computer because of important Windows updates. So I closed every application, including four Firefox windows, in mind, that Firefox restores every single window. The problem is, that this browser only saves 3 windows per default, not more! So be careful with how many windows you work, respectively how many windows you want Firefox to restore!
But there is a solution for this »problem«: you can access the advanced settings by entering about:config in the location bar and search for browser.sessionstore.max_windows_undo. Here you can set a custom value, so that firefox memorizes more windows :)