Desktop size suddenly shrunk.

I left my room for ~15 minutes, and when I came back, the desktop window's size collapsed to fill only a small window on the screen, leaving a large black area around the desktop, and a message was displayed saying that because my computer's memory was low it wanted me to close firefox.
I understand my system's memory might be low, but how do I revert the size of my desktop back to normal? Changing the resolution does not solve this issue.

Did you try restarting your computer?

Did you try restarting your computer?

I'll try once I'm done shifting some stuff (that's admittedly large) to an external hard drive.