Firefox: use ESR, also on Windows, that's usually a good choice. ESR won't crash, unless there is something wrong with a system dependency.
One of my favorites is Zotero, really nice to synchronize reserach over all devices, and it takes the hassle out of quote references, it can format those in dozens of standard formats, depending on what it's for.
If you're ongtk, guake, if you're on qt, yakuake. If you're on xfce, you don't need it because it's already there: a hotkey scroll-down terminal, the mother of all efficiency!
Eclipse with the plugins you need.
Zen
Digikam (qt) / Darktable (gtk) (digikam is way better)
Xsane
Audacious (gtk)
youtube-dl (because you won't have adobe flash or chromium-pepper since you're 100% FLOSS)
spice
eagle
LaTex (also a huge classic)
Xonotic
Minetest
Urban terror
Flightgear
Hamster (gtk, time tracker, best used with the gnome extension, hugely efficient!), for Qt, use the time tracker from the KDE PIM functionality, could be working with akonadi or not, depending on what KDE you're on
caffeine (the Gnome extension if you're on Gnome, on KDE the functionality is built in by default)
I don't know, what do you need an application for? Tell me what for and I'll tell you what I use for that lol
PS: I've been using Linux and FOSS as main software since 1996, with very little closed source use besides it. So 28 days is nothing in comparison to 18 years lol, it'll be dead easy, and you'll never want to turn back.