I used to use FileMaker back in the day... when web publishing was primitive at best.

Do you know whether there is a publicly viewable demo site online that a volunteer could use to examine this problem? I suspect it might be hard for you to provide enough detail just with text. Or perhaps you can post a screen recording illustrating the problem.

FileMaker WebDirect isn't supported in Firefox, according to its documentation for this product/feature: https://fmhelp.filemaker.com/docs/13/en/fm13_webdirect_guide.pdf (p28).