• APK file: Tap on the downloaded APK file, and follow the on-screen prompts to complete the installation.
• XAPK file: It's a compressed package containing an APK file and additional data files. You need a XAPK installer to install it. If you're using MEmu Android Emulator on PC, simply install the XAPK file directly into MEmu.

Yes, you can use Firefox Nightly for Developers on your PC using MEmu Android Emulator. After installing MEmu, you can either drag and drop the downloaded APK/XAPK file into the emulator, or open MEmu and search for Firefox Nightly for Developers to install it directly.