Firefox pdf viewer unable to save document

when I am clicking on a pdf document i.e.*******/remote.php/webdav/Nextcloud%20Manual.pdf (Browser firefox)
I get the pdf preview of the file.
When I click to the “save document” symbol nothing is happening.
The very same thing is working fine with internet explorer 11.
When I am changing my pdf - settings in firefox from “preview in firefox” to “open with acrobat reader” then this change is ignored when using nextcloud.
I saw in Nextcloud_Server_Administration_Manual.pdf :

Note: Technically Nextcloud can also generate the previews of other file types such as PDF, SVG or various office
documents. Due to security concerns those providers have been disabled by default and are considered unsupported.
While those providers are still available, we discourage enabling them, and they are not documented.
Can someone explain what is going on there?

There is a bug report: