What OS does your PC run?

In some cases, Firefox may automatically resume your previous session if it was left running when the system restarted.

There's an article for this for Windows 10: Firefox starts automatically when I restart Windows

For Linux, if the system signals Firefox that the reason it needs to shut downis an os-restart, then Firefox sets a flag to restore the previous session automatically. (Based on brief investigation for recent threads; I don't know whether this is officially documented anywhere.)

Of course, crash recovery also is a possibility. I didn't mean to discount that.

I'm running Windows 10. It doesn't auto start when I turn on my PC, so I don't think that's causing it. I'm currently thinking that it thought it got force shutdown, or didn't get shutdown propertly which is why it happened.

Since it's happened twice so far I thought I'd post here though

Hello tolly67p,

Prevent Firefox from starting automatically

• Type about:config in the address bar and press Enter.
• A warning page may appear. Click Accept the Risk and Continue to continue to the about:config page.
• Search for toolkit.winRegisterApplicationRestart.
• Click the aboutconfig-ToggleButton button next to it to set its value to false.

It will Surely Work. Thank You.

Firefox starts automatically when I restart Windows

If you use Session Restore (Restore Previous Session) as the startup setting then Firefox will restore open windows and restore login cookies as part of the data stored in sessionstore.jsonlz4.
Even data can be stored from closed windows and tabs (History -> Recently Closed Tabs/Windows).
Restoring or continuing download states might be part of open tabs or part of the history.

Are you using "Clear history when Firefox closes" to clear personal data ?

I think I've worked it out. Turns out I just turned an option off in the extension