The built-in PDF viewers provided by Mozilla Firefox and Google Chrome do not support XFA-based PDF forms (fillable PDF's). Therefore, XFA-based PDF forms often do not behave properly in these browsers. We have also seen similar issues with Microsoft Edge. The limitation is in the software provided by the browser; there are two common work-a-rounds:
1) Download the PDF to your PC, and open it with Adobe Reader. You should then be able to fill it out and save it.
2) Change the default PDF viewer used by your browser to Adobe Reader. The Adobe website provides information about this work-a-round for Firefox and Chrome. For Edge, search the internet for "disable edge pdf viewer".