You select which features you want to enable and disable and in the end you get a download link for a zip-file with your profile template. You can for example disable some functions, which send data to Mozilla and Google, or disable several annoying Firefox functions like Mozilla Hello or the Pocket integration.
Note: This is not an endorsement.
Disclaimer: some of this steps might mildly detriment user experience, security is always a convenience trade-off. Some of the steps may require extra research, always follow trusted sources.
Note: Many useful tips, though each should be carefully evaluated in terms of your own computer usage and needed functionality.