What are your settings for PDF files in Settings>General?

My setting for Portable Document Format (PDF) is okular, and it doesn't work. I cannot open any PDF/pdf attachments from Thunderbird. This used to work.

I set my Ubuntu default for PDF files to okular. After that, if I open Nautilus (file manager) and double-click, the file opens in okular. I also set it for .PDF files, just to make sure. Double-clicking the file in Nautilus opens the PDF in okular.

It doesn't matter whether I set Thunderbird's Files & Attachment setting for PDF to System Default or /usr/bin/okular. NEITHER SETTING WORKS!

And, yes, I restarted Thunderbird after experimenting with all the variations of the settings.

Ubuntu Linux 24.10