Using a Linux Debian desktop.
Used to be able to open PDF email attachments in Opera and Firefox-esr browsers without downloading them.
Suddenly no longer able to open this type of attachment in Opera or Firefox-esr without downloading the attachments.
Also not able to open PDF files from various websites.
No changes at all have been made to the Debian system or any software on that system.
How can this problem be solved, please ?
It is not an option to download attachments on a regular basis.
Firefox Extended Support Release (ESR) seems a bit overkill for a domestic environment - or is this a commercial environment (which is fine, by the way). Since FF ESR is designed for a particular kind of user, this might be a contributory factor to your problems.
You say you are using a Linux Debian system.
Can you provide a bit more information about your system? Like which Debian-based OS (and version) you are using (Ubuntu, Mint…) or is it Debian itself? This might help people to advise you.
Which PDF reader do you have? And have you tried another one?
According to PDF - Debian Wiki Debian comes with several PDF readers in the package.