how secure is this app?

Yes, Firefox is one of the most secure and private major browsers available. To secure your device and maximize your privacy: 1. Enable "Strict" Enhanced Tracking Protection * Click the Shield icon to the left of the address bar. * Select Privicy Settings (or go to Settings > Privacy & Security). * Under "Enhanced Tracking Protection," change the setting from Standard to Strict. This will block almost all hidden trackers, though it may occasionally cause some websites to look slightly broken. 2. Tell websites not to "Sell or Share" your data * In the Privacy & Security menu: * Scroll down to the Website Privacy Preferences section. * Check the box that says: "Tell websites not to sell or share my data". 3. Secure your DNS. This prevents your Internet Provider from seeing which websites you are visiting: * In Settings, search for "DNS". * Under DNS over HTTPS, select Max Protection.