Follow these simple steps to disable telemetry in Firefox:

1. Download user.js file from this repo
2. Find your profile folder there about:support
3. Move user.js to profile folder
4. Restart firefox

Special thanks to K3V1991 for the idea. You can find the original repository here - https://github.com/K3V1991/Disable-Firefox-Telemetry-and-Data-Collection