If your tracking protection setting in about:preferences#privacy is set to Custom, but your custom options exactly match the ones in the Standard settings, Firefox will switch back to the Standard. You will not lose the privacy options you have selected since your custom settings matched the standard ones.

Change back to Custom settings

You can change back to Custom settings at any time and choose options that are different from the Standard settings. To learn more, see Adjust your global Enhanced Tracking Protection settingsthe Content blocking article under Custom settings.