ESR102: ensure download improvements don't exacerbate disk leaks

@richard FYI and also labels please: FF102-esr, Disk Leak , @pierov FYI

ESR102 introduces changes to download behavior and use of /tmp/

• e.g. see this very noisy 1738574 and just skip to comments by Gijs
• btw, that bugzilla introduces a new pref to revert the behavior

Note: https://bugzilla.mozilla.org/show_bug.cgi?id=1738574#c40 from Gijs

If you pick "open with Firefox" as the default action for PDFs, we'll open them directly from the internet in almost all cases (there are some edgecases to be worked out, cf. bug 1742648), without storing anything on disk in the first place.

So to be clear, we tried not to change the default action for PDFs. We changed it for most other mimetypes, but not PDFs (nor any other filetypes set to "open in Firefox" by default)

We should make sure that any download improvements since the last ESR, don't create disk leaks. IDK if PB mode does anything special here. I haven't worked through all the variables, and it gets a little convoluted - hence opening this ticket to make sure we address it