hi marcoporcellini, firefox won't bring back the old options interface - the browser.preferences.inContent preference was just in place temporarily for a transitional period. now that there were new features and settings going into preferences and old windowed interface was no longer maintained, its users would have missed out on them, so the support has been totally dropped in firefox 42.

if you still miss the old styling - the classic theme restorer addon from https://addons.mozilla.org/firefox/addon/classicthemerestorer/ contains some tweaks to make the options-styling more familiar...

You can possibly create a button on the Bookmarks Toolbar to the about:preferences page and hold the Shift key and left-click to open the page in a new window.

Thanks, both useful suggestions. It's a real shame they have discontinued this as a core part of Firefox, I'm sure new features could have been integrated into a floating window just as easily as in-browser (in fact they have been in the Classic Theme Restorer tweaks). Also, having the same tab within Preferences pop-up every time was useful as you tend to have the same requirements when you use it (such as turning cookies on and off etc).

Still, thanks very much for the feedback and advice, I've used Classic Theme Restorer in the end as that's the nearest to what I wanted.