VAS appears incorrectly in printed PDF forms (firefox)

[malcjackson]

This is the same issue described in #41 but specific to Firefox. 671 seems to have resolved it in other browsers.:

Describe the bug
When printing a form as PDF, if that form contains a VAS item, the VAS appears rendered without tickmarks and other visual issues.

To Reproduce
Open a form with a VAS item, fill in data, and print it to PDF in firefox, selecting the "Background Graphics" option. The printed PDF will have a VAS without a central line or tickmarks.

FFVasError.txt