Make sure you do not close open windows, but close the Firefox application (Firefox -> Quit) while all window(s) are still open because otherwise session restore won't work properly.

In case you use "Clear history when Firefox closes".

• do not clear the Browsing History

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

Thanks cor-el, you're right, clicking the close button means that the session won't be restored. Cmd-Q is needed to allow the session to restore.

This is bullshit! I can't save my settings if I close the browser by one-clicking on the top right X?

Why would they do this????

I really like using Firefox, but this may be the last straw.....

Hi Susana L. Gallardo

Are you on Mac as well ?

On Mac this works differently because you can close all open windows and still have the menu bar visible. If you have closed all open windows and then close (Quit) Firefox then there is nothing to restore and you can possible look for "Recently Closed Tabs and Windows".

On other platforms it can happen that you use the close X, but still have other windows open. This can be an unnoticed or forgotten browser window or another standalone window like the Library (bookmarks and history manager). Only by using the menu item to Exit/Quit Firefox you can make sure that you close Firefox properly. The close X first closes this active window and then check whether there are other windows open and if found sets focus to the last used window. Only if no other windows are found open then Firefox is closed and session data flushed to the profile folder. Closing Firefox via the close X is less safe than using the menu item.

Susana L. Gallardo said

I can't save my settings if I close the browser by one-clicking on the top right X?

Hi Susana, what settings are you losing? You shouldn't lose settings just from closing Firefox. See: How to fix preferences that won't save.