I don't have any problem with change - I've spent some time changing Firefox the way I want it. I do this partly because I have a number of add-ons set up the way I like, partly because I prefer it laid out the way I want and partly because the onset of cataracts means that I'm obliged to customize colours and font sizes and set up short-cuts to deal with sites with unreadable colour schemes.
Ironically I was actually reading this thread as the update downloaded. Just as well really because the hour or so getting Firefox back to how I like it would have taken far longer without the useful advice. Cheers commentards. (It wouldn't have taken as long as that if one add-on ('RSS Icon') hadn't choked on the upgrade and stuck every available icon into the address bar). Now if my eyesight was worse than it already is, or I hadn't been pointed to the add-ons I needed, then I would still be struggling with things.
I'm resigned to Firefox changing things periodically and my having to do some housekeeping. At least unlike Opera or Chromium I have that choice. And it's good to have a tidy up once in a while. Nonetheless there are a couple of aspects of this upgrade that are more than a little tiresome. I understand perfectly well that Mozilla feels I should prefer tabs on top - but I don't. If an upgrade is going to retain some of the choices I have made in the past why does it always override others - tabs on top being a case in point. Similarly colours. Having customized the default background colour because of my eyesight why does it insist on resetting it.
And most tiresome of all if you're going to remove the Add-On bar and any custom toolbars that have been set up why keep all the shortcuts they contain and stick them into the remaining toolbars - in my case making the browser temporarily unusable while I got to grips with the new customization screen. Very dumb.