@chocolatkey
Since I posted this, I attached a larger, external monitor. There was also a reader or Firefox (or both?) update that now lets me "zoom" the image and fiddle with the image size to make it more comfortable. It's really wonky, and I would still prefer a more explicit image size control (still a fan of setting image width to browser window width).
This is also entirely based on personal preferences, comfort level, and hardware, so not sure how well this will come across in screen shots.
I expanded FF on Windows to vertically fill the screen and adjusted the browser width. I used windows snipping screenshot of the window, so the image size is the window size: too big and too small.
The "too big" image is large enough that I still have to physically move my head back to comfortably view. On my monitor, the manga image is about 9.5 inches across.
If I make the window width 3 pixels smaller, the manga image drops to 6.75 inches across. This is readable on my larger monitor, but I wish it was just a bit larger.