Hi

Firefox for Android is a single application on a mobile device and does not provide protection for that device, just the web browsing you do through Firefox for Android.

Without knowing what app or tool is being used, it is difficult to appreciate the full picture or suggest a solution, but you may wish to consult your telephony service provider or a dedicated Android forum for help with this issue.

If someone can access your phone at close range, they might be connecting via Bluetooth. Your device should be skeptical of connecting to new Bluetooth devices, but if you've connected once before, perhaps it trusts them now. I suggest researching Bluetooth settings to clear out any trusted devices from the list that are not completely in your control.