//Copy the following code into the browser's developer console on a Salesforce object's FLS edit page for an indivisual profile.
//This will check all the visible checkboxes for you, great for very field heavy objects
for(var a=document.querySelectorAll("[title='Visible']"),i=0;i<a.length;i++)"checkbox"==a[i].type&&(a[i].checked=!0);