FIRECalc does not store any information you enter on this page.
That's great. Where does it process the data?
Is FIRECalc based on JavaScript or does it submit any data to PHP?
I'm guessing that since the results page has a .php ending that the input data gets sent to a PHP script at some point.
You might take a look at the Privacy link at the bottom right of the FIREcalc pages to see if that addresses your concerns (https://firecalc.com/privacy.php).