I do have a new icon in the place you refer to; the (resettable) zoom level currently at 90%
Not sure what the full default layout is with the bar, but I customized mine a long time ago. The situation is there are a number of icons you can have on the Toolbar, and they can sit on either side of the URL address window. The Search window is usually to the right of the URL window, but I never use it so it goes. For years I've had the zoom level icon on the far left of the bar with most of the common use icons between that and the URL window, then the Preferences icon with a co0uple more between the URL window and the three bar Open Menu icon on the far right. Now there's a nice gap between the Refresh icon on the right hand end of the URL window and the Preferences Icon, so if I miss the Refresh icon by a little bit there's nothing there to activate. Also, the right hand end of the URL window is empty and nothing there to activate, and if the URL is long enough to fill it, it still won't activate if I click on the end of the URL Thus there was a nice 'safe' zone around the refresh icon - which is good due to how small it is on the 4K screen set at 3840 x 2160, so any slight jittering of the mouse didn't cause any trouble.
However, the icon for the new feature of the Firefox Reader View is in the right hand edge of the URL window (until I killed it) snugged right up against the Refresh icon, so if you slid off one you went straight into the other.
Why they didn't have it as another icon you can add or remove via the customization process is beyond me, because that's the most logical way to do it. But they didn't. I checked the FF customization and preferences and add-ons before I started this thread. Since there was no visible way to turn it off in FF, and of the eight websites I had open with windows the two SoL windows were the only ones showing it, I thought it had to be a Sol related thing similar to the one that allows you to adjust the screen setting for the story pages because they both appeared on the same pages most of the time.
Well, I've found where to turn it off, but it wasn't easy. And then it was only after someone could confirm it was a FF issue and not a SoL one.