browser.sessionstore.max_tabs_undo is by default 10 and browser.sessionstore.max_windows_undo has a default value of 3, so that is expected behavior.
I'm afraid that the tabs from that other window are lost and you will have to check the history to see if you can reconstruct them.

Thank you for ending the suspense. Unfortunately, I have in an interstate move and had been accumulating the Tabs in that Window for later reading for sometime. I could probably never backtrack to find them.

Yes, with 3, that would be the expected behavior that I have encountered. I may have mis-read, but I thought I had read in some help note that 'max_windows' was supposed to be 10.

Regardless, given the way all these various marketers, etc., spawn 'push' windows that the user is not aware of until too late, the default should certainly be 5 or >, if not 10.

I have now set mine for 10. I hope Mozillers will take note. Thanks!

How you shut down Firefox makes a difference for this feature.

If you right-click the taskbar icon and choose Close all windows, then Windows literally instructs Firefox to close each window in your session and Firefox will, by default, only recall the last window closed and the 3 closed before that. The order of closing is based on the sequence in which they were opened, or some other factor controlled by Windows, completely unrelated to importance or usefulness.

For better results, always use Exit, either from the File menu or the orange Firefox button. Then all of those windows are considered to have been open at the time of shutdown and you lose nothing, even with the default setting.

How long did you have that set of tabs open? If it was for an extended period, e.g., a week or two, it's possible that Windows saved a snapshot of your sessionstore.js file in a restore point.

To see whether Windows has earlier copies, you can check the Previous Versions tab in the Properties dialog for the file. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Scroll down to the files whose names start with sessionstore. You should find at least sessionstore.js and sessionstore.bak. If the .js extension does not appear on sessionstore, you should turn off the Windows feature that hides file extensions from you, so that you can work with file names as accurately as possible. This Microsoft support article has the steps: http://support.microsoft.com/kb/865219.

Right-click sessionstore.js > Properties > Previous Versions. This may take a while to load. Any earlier files look promising?

Before restoring anything:

(1) Exit Firefox and allow time for final file updates

(2) Make backup copies of current sessionstore files to somewhere like your Documents folder for possible later restoration

That is useful info. I have always been clicking the Red X in each Window.

Yes, I can find several previous sessionstore files, but when opening any to review, each fails to open with a 'Microsoft JScript compilation error'.

Hi Westtrekker, for your security, do not double-click .js files, since some may be designed to operate on Windows itself (for example, Windows scripts can change registry settings, delete files, etc.)

To read the file contents, you will need to either:

• right-click the file > Edit (should open automatically)
• right-click the file > Open With > choose a text editor