Since a few versions ago, PDFs open in the same tab by default. Before that, sometimes they would open in the same tab, sometimes they would open in a new tab, and sometimes I would be asked what I wanted to do with it.
I can imagine that the inconsistency of the old approach is less than ideal, but the new situation is significantly worse in many situations.
E.g.
* when someone sends you a pdf in webwhatsapp, it's much nicer if the pdf opens in a new tab instead of in webwhatsapp (the behaviour here is also inconsistent with opening other links from webwhatsapp, as non-pdf download links do open in a new tab)
* when clicking on a pdf link in a pinned tab, it's much more practical if the pdf opens in a new tab instead of now having a pinned pdf file
* when downloading your working document from a site like overleaf.com, you don't want the editor to be replaced by the pdf viewer
In all of these situations, opening the pdf file in the same tab is undesirable. This would be fixed by adding an option 'open in new tab' to the actions menu in about:preferences, or by changing the open in firefox option to open the downloaded file in a new tab.
