Why does the page zoom out when I press the Ctrl key without scrolling or pressing any other key?

That is the default action for the mouse scroll wheel.

You can set mousewheel.with_*.action prefs like mousewheel.withcontrol.action (on Mac mousewheel.with_meta.action) to 0 on the about:config page to prevent their default action if the Ctrl/Command key is pressed.

0 means "Do nothing" 1 means "Scroll contents" 2 means "Go back or forward in the history" 3 means "Zoom in or out the contents"

• https://wiki.mozilla.org/Gecko:Mouse_Wheel_Scrolling

I am literally pressing the control key only, but your response is regarding the Ctrl key + mouse scrolling. I'm not using the mouse wheel. When I press the Ctrl key (without touching any other key and without scrolling on the mouse) the page zooms out.

It sounds that Firefox still thinks that the wheel is rotated or that the plus button is pressed.

Did you close and restart Firefox or possibly rebooted the computer?

Try to start Firefox in Safe Mode to see if that has effect.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode