fba827 and frankthedm - thanks you two, that was it and now it is fixed and I am very happy - no more side scrolling.
Now I'm wondering what I did to cause that to happen.
For me, when ever I unintentionally do it, it's from holding down control and then using the scroll wheel on my mouse
(i hold down control frequently on Firefox to force links to open in a new tab; so if i'm still holding down control when i move the mouse's scroll wheel i get the zoom effect).