Finally remembered that I have to specify the “experimental” target to get
Apt to install the newer Iceweasel version on my Debian Unstable system.
I see lots of new HTML-side goodies in CSS, Canvas, video and other stuff.
On the user side, I noticed the changed behaviour in opening new tabs next
to the one I’m currently looking at, instead of at the end of the row in the
window. As someone who likes to keep lots of windows open, with lots and
lots of tabs in them ... I actually like the new behaviour.