Pdf reader fails to open in webview post 19.0.4 -> 20 upgrade

Nextcloud version (eg, 18.0.2): 20.0.0
Operating system and version (eg, Ubuntu 20.04): Debian 9.12
Apache or nginx version (eg, Apache 2.4.25): 2.4.25-3+deb9u9
PHP version (eg, 7.1): 7.4.6

The issue you are facing:
Using default pdf viewer pre and post upgrade, pdfs no longer open in webview via Firefox (on Windows & Linux).

Is this the first time you’ve seen this error? (Y/N): Y

The output of your Nextcloud log in Admin > Logging:

No warnings or errors outputted to logs

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):


The output of your Apache/nginx/system log in /var/log/____: