Firefox xlsx viewer incorrectly displays output forms

[JeltevanBoheemen]

To replicate:
Analyse a transcript with ASTA on Firefox, choose form output. The file is generated, and the user can click the completed download to view it in their browser (default behaviour). In the browser, cells miss background color and some values are missing.
Opening the file offline, e.g. in Excel, shows the correct document.

Investigate if there is some low-hanging-fruit solution to making the xlsx files work in the browser viewer. If not: indicate to the user they should not open the form in their browser.