This happens only when another program is deleting the temporary files stored in your computer. Programs such as Registry Mechanics or Resident Shield or some antivirus with extra options remove unused files. Or if you are cleaning your files for any privacy reason.

Also try remember what happened when it stopped working. Did you install anything else? (i don't know how advanced you are in computers but it could be another program conflict).

It could be also another problem if my answer did not give you any solution. Hope someone will answer it. Regards.

This can happen if Firefox isn't closing properly or if you close windows in the wrong order (i.e. you didn't notice a pop-up window).

Use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar.

Co-rel got a point but should Firefox remember the order? or should disregard unclosed windows that are in the background and forced closed by closing the main window??