It's the flatpak version and, sadly, it doesn't allow me to open attachments. When I click on the file, it brings me to a screen where it proposes to use the system handler to open the file and then nothing happens.
Apparently, this is a common issue related to flatpak and its permissions, so I tried to circumvent it by trying to open documents inside Thunderbird (as I remember Thunderbird being able to open PDFs) but it didn't work as it's only telling me that Firefox is already running.
Does anyone know a workaround as for now it forces me to save documents before being able to open them?
Yeah for sure, thats the pdf.js also available in Firefox.
Why do you specifically want this in TB?
Firefox Flatpak worked normally via the portal for opening the PDF. I think Thunderbird defaults to sending a file as an attachment to a new empty mail.