Try this:

https://support.mozilla.org/en-US/kb/restore-previous-session

I don't think that is possible. You normally do this by closing and restarting Firefox to make Firefox save session data to sessionstore.jsonlz4 in the profile folder. If you would close all windows without closing Firefox then you do not free all memory and otherwise the memory in use might be fragmented and there is no way to defragment this apart from closing Firefox and restoring closed window might not work reliably (Firefox remembers by default three closed window).

Try this: https://support.mozilla.org/en-US/kb/restore-previous-session

Thank you for your answer. Sorry for responding late but I wasn't home for a few days. I don't dare to do this because Firefox will think it's a new session. Will "restore all tabs in all windows" be suggested in menu ?

I don't think that is possible. You normally do this by closing and restarting Firefox to make Firefox save session data to sessionstore.jsonlz4 in the profile folder. If you would close all windows without closing Firefox then you do not free all memory and otherwise the memory in use might be fragmented and there is no way to defragment this apart from closing Firefox and restoring closed window might not work reliably (Firefox remembers by default three closed window).

Thank you for your answer. Sorry for responding late but I wasn't home for a few days. I know "sessionstore.jsonlz4" and i've already used it in case of emergency. I would like to close all Firefox windows and then Firefox itself. I wish it could be possible to make it read "sessionstore.jsonlz4" automatically at next start up. Do you think there is a trick with a script or something ?

You can close and restore individual windows as long as you leave one window open. You can also close and restore tabs; I do that to reduce the use of ram. You restore with History > Recently Closed Tabs > | Recently Closed Windows >. Tabs use memory inequally. To see how each is affecting performance, use Help > Task Manager (or enter about:performance in the url bar).

You can close and restore individual windows as long as you leave one window open. You can also close and restore tabs; I do that to reduce the use of ram. You restore with History > Recently Closed Tabs > | Recently Closed Windows >. Tabs use memory inequally. To see how each is affecting performance, use Help > Task Manager (or enter about:performance in the url bar).

Thank you very much. I didn't noticed that restore all windows was possible in history. I think this is the best solution for me at the moment. I will leave one window open and do this.