Firefox Take a Screenshot: download failed - blob resource

The "Take a Screenshot" facility in Firefox always used to work fine in Sandboxie before the latest release of Windows 1903. However now, when saving the file as a download, it fails - "blob resource". The only solution is to shut down the browsing session, then open a new one with Sandboxie disabled and repeat the exercise.

Any ideas what may be causing this, and is there a solution?