I have tested and confirmed the existence of the problem in Firefox and also under Windows.
The problem is not isolated to reports, but to all pages where the attachment is downloaded (wizard, import/export, ...).
After long debugging, I have found partially to the cause. The problem is related to monitoring and the long-polling used in it. If I disable long-polling (using property: 'idm.pub.app.long-polling.enabled'), then the problem does not happen.
I have not been able to figure out the specific cause, but it probably has to do with the fact that Firefox is doing a 'redirect' at the moment (throwing away the connection history). I have not been able to fix this situation (I don't think this is possible in our code).
As a workaround I found, use a new tab in the download links. From a UX perspective this is less convenient (blink/redirect), but it works in Firefox.
I have made the adjustment (hopefully) on all product sites that download the attachment.
target="_blank"
rel="noopener noreferrer"
download
Commit: https://github.com/bcvsolutions/CzechIdMng/commit/6996fb82b12c60754f2ccda0285ddaa984c7380c
