Your Period Data Should Be Private: "We recommend using Euki."

Mozilla claims open source but that doesn't appear to actually be true - the previous version of the app was released under GPL 3 2 years ago. I can find no claim on their website that the new revision is open source or any way to access the source, and they're not on F-Droid. IMO that's a no go, no matter how thorough Mozilla was in their 3 hours of testing.

I recommend drip. which is actually open source, fully local and available on F-Droid.