I'm assuming this affects all applications and not just Firefox...

I haven't heard of that feature. Is it a convertible laptop/tablet??

You might check the Fn key combinations, often found printed on the Function keys, to see whether one of those looks like it rotates the display.

If you can't find the key combination, it would be helpful to know what kind of device it is in case anyone has experience with it.

thank you for that. Actually it was an accidental mistake on Windows Microsoft. Went to Display and found the combination of keys: window key and L for reverting back to landscape.