Firefox saves the current session automatically when you close Firefox and doesn't show the dialog asking to save the current session. Use "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit) if you want to restore multiple windows or otherwise have problems with restoring tabs. You can use "History > Restore Previous Session" to get the previous session at any time.

There is also a "Restore Previous Session" button on the default home page (about:home).

In case you are using "Clear history when Firefox closes":

• do not clear the Browsing History

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

• Session Manager: https://addons.mozilla.org/firefox/addon/session-manager/

What you can do is to Bookmark that page. You can save the bookmark to a sub-directory of your choise, or create a new sub-directory.

I wish you people would answer the OP's question rather than suggesting something he has NOT asked you.

Wanting to save tabs is a completely different request to Firefox reloading the last session.

You used to be able to right click the tab toolbar and save all tabs to Bookmarks.

If anyone knows how to do this I would be grateful.

So the answer to the OP is

Right click the tab toolbar and select

Bookmark all Tabs