random code comments on content_script.js:

The indentation is crazy (subjective). Use spaces not tabs, choose 2 or 4 spaces. I prefer two, but don't really care.

there's a lot of repetition: All of the textboxes could be initialized with a function that took key/value pairs for attributes. They are all set to float right.

don't write stuff like document.getElementById("popOver") over and over, it's ugly. Do it once and assign it to a local variable. This is for style, not efficiency.

I didn't read the getTextSelection() function thoroughly, but in my experience, stuff that looks like that never works quite right. Attacking date inputs is tough, and will probably require a separate, tested library.