What appearing on the left side?
Perhaps the Side Bar is open. Any add-ons that affect or add to the Side Bar features?

Beyond those items, please provide a screenshot of the Firefox browser window.
https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

Windows has some keyboard and mouse shortcuts that can size a window to half the screen or "snap" it into place. For example, holding down the "Windows" key on the keyboard and tapping the right arrow key will fit the window to the right half of the screen.

In most cases you can manually resize the window by dragging borders/corners, or maximize it to resolve the issue. Is yours locked into place, and non-resizable?

I can resize it by dragging the corners or using a private window but it doesn't last. Every time I first open Firefox it goes to half size and needs to be resized. Is there a way to have the resize be permanent?

Hmm, I don't know why Windows/Firefox remembers that size. Usually it should remember the sizes of the windows open at the time it closes. ??

I was unsuccessful in making a screen shot. But the half firefox screen is on the windows desktop screen. There is no other application showing. Just the half firefox on the desktop All of the Firefox sides are closed.

When I click on the Firefox icon it opens in full screen and then immediately shrinks to half screen. Right now it's in full screen because I stretched the Firefox screen with the edge. When I close it, Firefox will open as described above: full screen to half.

Could you try dragging the window to approximately 1" down and 1" over from the upper left corner of the screen, and then resize it from there? Trying to think of a way to break the right-size habit.

It's fixed and I'm not 100% sure what worked. It's been stuck for several days and friends have tried to help with restoring without success. I tried all the suggestions made here and at one point it switched to a half screen horizontal. After I turned that off and restarted it came back to full screen.

Thanks to all of you who made suggestions