Well to make it simple using about:config from the browser (very slow BTW)
I just inserted in the "Name" and in the "value" box and hit "Set Preference".

I walked away for a cup of coffee (did I say slow), came back and found that the font size was now changed on the page to a more readable size. I closed the page and restarted the browser.

Now my browser opens and displays text by default at what I used to have to zoom to 120% to achieve.
A much better user experience.

Previously I had found that 120% was about the max to use before the "fit to width" feature started moving web objects arround...