When I'm saving downloaded files through firefox, attaching files to a mail, opening files with programs, etc. then I have to use the program's own file browser (in this case Firefox's).
My question is, is there a way I can globally add more directories?
In this case with Firefox specifically, to the left column along with Recent/Home/Desktop.
p.s. I'm using Arch Linux with i3wm and normally I use the terminal rather than any GUI file manager/browser
In my experience, adding folder shortcuts in Thunar (XFCE's file manager) makes them also appear in Firefox. I believe it's some sort of config shared among all GTK apps. So you can try installing Thunar and adding shortcuts in the left pane, then see if they show up in Firefox and other apps.