I am running both Firefox and Thunderbird in firejail, on Linux Mint 19.1.
By coincidence, two people recently sent me emails with attached PDFs, but they were handled differently. The emails came through separate accounts (gmail and GMX).
For the first one, the link appeared in the body of the message. Simply clicking the link caused it to open in (already running) Firefox.
For the second one, there was a "1 attachment" message in the status bar of Thunderbird. I was unable to view this PDF without first downloading it, then explicitly opening it in a viewer.
I understand the basics of what firejail does, at least to the extent of hiding your files. What I don't understand is how two attachments can be treated so differently. Is this to do with how the sender created the messages, or is it something I can control?