How to exit a stuck/frozen post mission menu in Warframe

If the timer runs out before you get to extraction, you only lose the credit reward. All mods, resources, mission reward mods and items will be kept. The next mission is also unlocked.

(As of Update 9)

They're slotted as other mods.

Your frame's mod slots will have four slots with an '=' in the upper right corner. These indicate that they are Polarized so that a mod slotted there will only require half as many ranks to slot as its cost would indicate, and the '=' type marking is reserved for frame powers.

If you're only just starting out, you won't be able to slot all four of them yet (you will have all four in your inventory; they're gained at the same time you acquire the frame that uses them), but leveling up your Frame increases the number of slot ranks available.