This may be true on Windows now as well, since some users have reported that right-clicking the stack of Firefox icons on the Task Bar and using "Close all windows" causes some windows/tabs to be lost.

A couple of notes for future reference in case you accidentally click the red X out of habit:

(1) Firefox will remember up to 10 closed tabs and 3 closed windows. After you choose Restore Previous Session from the History menu (or if you have Firefox restore your session automatically), you can use these lists:

• History > Recently Closed Tabs
• History > Recently Closed Windows

(2) The default values of 10 and 3 can be increased if you tend to have more than 10 tabs open on a regular basis. Here's how:

• In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

• In the search box above the list, type or paste sess and pause while the list is filtered

• Double-click the browser.sessionstore.max_tabs_undo preference and enter your preferred number of tabs (I have 20)

• Double-click the browser.sessionstore.max_windows_undo preference and enter your preferred number of windows (I have 10)

This a known issue with Firefox. By clicking the close button on the window, you are forcing Firefox to close right away. This prevents Firefox from saving you closed tabs and will say that Firefox has crashed.
Pressing the Exit button in the Firefox menu will allow Firefox to close as soon as it has saved the required information.

Many people don't know this, but whenever a program has an exit button, this is the button that should be used. The exit buttons are added to programs because the developer(s) have noticed that the program is not running properly when the window close button is used.

jscher2000 thanks a lot for the information! But option 1 doesn't work because the session isn't saved. The 'restore previous session' button is blanked out.

I can only save my session with CMD+Q! Is there a other way to exit firefox with the red (x) button and not lose my session?

On Mac, if you close a window then the Firefox application is still running, so can use History > Recently Closed Tabs/Windows to reopen closed tabs, but if you first close the windows and then quit Firefox then you open tabs aren't saved. So you need to close Firefox while the windows and tabs are still open.

Hi MastarLin, when Restore Previous Session is grayed, this could indicate that your session never ended (see cor-el's post above) or that your session resumed automatically (this may happen after a crash).

Or it could indicate that Firefox lost your previous session data and isn't able to restore it. (Insert sad face here...)

I agree to the post of cor-el. Can reproduce my problem with this information (if you have restore session on Firefox startup selected).

So here are some tips:

1. Never close the Firefox window if you shutdown the system. Keep Firefox open, minimized or close it with (CMD+Q).
2. Do not quit Firefox (CMD+Q) after you have closed the windows with the red x button.

Hey,

I already am doing that now... but it's annoying! Why can't I just press the red x button!!!!! :( Mozilla really needs to fix this.

This is not something that Mozilla can fix.
Please read my above post.

On Mac the red X button closes the current window and this leaves the Firefox application with the menu bar still running in case you close the last open window.
If no windows are open when you quit the Firefox application then there will be no windows and tabs to restore.
On Windows and Linux, closing the last window would close Firefox, but only the last window would be restored the next time and other windows would get lost if you do not use Exit Firefox to close all open windows at once.