In case you use "Clear history when Firefox closes".

• do not clear the Browsing History

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• delete sessionstore.jsonlz4 and sessionstore.js

When sessionstore.jsonlz4 is deleted you lose pinned-tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. sessionstore.jsonlz4 is only present in the Firefox profile folder when Firefox is closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Note that on Mac closing Firefox can be more complicated because you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar.

• "3-bar" menu button -> Exit (Power button)
• Mac: Firefox -> Quit Firefox
• Windows: File -> Exit
• Linux: File -> Quit

> In case you use "Clear history when Firefox closes"

I don't.

> delete sessionstore.jsonlz4 and sessionstore.js

I looked in the profile folder and they weren't there. The fact that they were missing is indicative of the issue - if it's not saving my session, I'm guessing they won't be present.

sessionstore.jsonlz4 is only present in the main profile folder in "AppData\Roaming" when Firefox is closed. When Firefox is running then files in the sessionstore-backups folder are used.

Ok, so I've been doing some reading on this and I think I've just mis-understood how Firefox deals with restoring tabs.

Please tell me if this is correct...

1. If you exit Firefox in Mac via the menu (i.e. Firefox -> Quit Firefox), it saves all your tabs.
2. If you exit Firefox by any other means, it doesn't save your open tabs.

Is this right? This is different behaviour to what I'm used to (Chrome user before).

Also, why does Firefox (in the 2nd example), save pinned tabs?

The pinned tabs are in the session history file, so the question is what else is in there...

Firefox remembers up to three closed windows. However, instead of using the same shortcut you use for re-opening the last closed tab, as in Chrome, you need to use a different shortcut, or the menu.

First, did Firefox restore your previous session? Check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, then check for closed windows. Either:

• "3-bar" menu button > Library > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

Does Firefox list them there?

Each window has its own set of pinned tabs, so if you get the pinned tabs restored, but not unpinned tabs then this means the Restore Previous Session isn't enabled or isn't working properly, but sessions restore as such is working.

Your troubleshooting information list shows some tab related extensions with Undo Close Tab being enabled.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems