Go to menu ≡ -> Settings and turn off Open Firefox automatically when your computer starts up.

Thanks but that box is already unticked, I also turned browser.startup.windowsLaunchOnLogin.disableLaunchOnLoginPrompt to false

To disable Firefox from loading automatically when your computer restarts or boots up, follow these steps:

On Windows: Disable Firefox in Startup Programs:

Press Ctrl + Shift + Esc to open the Task Manager. Click the Startup tab. Find Mozilla Firefox in the list. Right-click on it and select Disable. Check Task Scheduler:

Press Windows + R, type taskschd.msc, and press Enter to open the Task Scheduler. In the left panel, navigate to Task Scheduler Library and check for any scheduled Firefox tasks. If you find one, right-click on it and select Disable. On macOS: Remove Firefox from Login Items:

Go to System Preferences > Users & Groups. Select your user account, then click the Login Items tab. Find Firefox in the list, select it, and click the minus (-) button to remove it. Check Firefox Preferences:

Open Firefox. Go to Menu (three lines in the upper right) > Settings > General. Under Startup, make sure "Restore previous session" is not checked, which could cause Firefox to reopen after restarting. These steps should prevent Firefox from loading automatically at startup.

Thanks I checked those. I still have the same problem. I have also tried the autoruns app from microsoft but that didn't help.