Virtual Kiosk blocking Firefox downloads

I have been using virtkiosk.exe for months now when accessing sensitive files and websites. My default browser is Firefox. Up until a week ago, I have always been able to download files via FF and save them to the sandboxed Downloads folder or directly to Shared Space. But following the last round of updates every download attempted via FF from within virtkiosk fails (pdf, mp3, json, ofx, &c…). However, the download succeeds when the same FF version (54.0.1) is run outside virtkiosk and also succeeds when downloaded via Chrome from within virtkiosk. I have not knowingly made any configuration changes and cannot find any explanation for this baffling change of behaviour in the log files. The current version of virtkiosk is, dated 11/7/17. All .cfgx files are dated 13/7/17.

Any suggestions?