Hello ChuckKelly,

Probably a rhetorical question, but : do you see a closing X on the window (left or right - you mention both)  ? If so : then you can click on the maximize button (upper right, between the <big>'-'</big> and the closing X).

(you may have inadvertently pressed the Windows key on your keyboard and the left or right arrow key ... )

If that's not it : could it be that you're using the Test Pilot's extension 'Side View'  ?

See : https://www.ghacks.net/2018/06/05/a-look-at-side-view-an-upcoming-firefox-test-pilot-project/

and : https://blog.mozilla.org/blog/2018/06/05/latest-firefox-test-pilot-experiments-custom-color-and-side-view/

https://addons.mozilla.org/en-US/firefox/addon/side-view/

Any chance that you are opening this forum via a saved bookmark?

If so, open Properties for that bookmark and un-check the Load this bookmark in the sidebar.

the-edmeister said

Any chance that you are opening this forum via a saved bookmark? If so, open Properties for that bookmark and un-check the Load this bookmark in the sidebar.

Good point  ! I didn't think to ask, because it would have to be an old(er) bookmark since that option to open a bookmarked page in the sidebar was removed.

That's could very well be it, though .....