I would consider the list of browsers updatable in stable branches, like MIME types and encoding aliases.  The alternate names for Mozilla browsers are really just strings in a list; xdg-open/x-www-browser/gvfs-open are a little more (need to use the right class i.e. BackgroundBrowser).  I think the goal of the backport rules is to 1) not compromise stability by adding new code 2) not break code that was valid in the previous bugfix version.  Adding encoding aliases or browsers seems much smaller to me than for example the cStringIO unicode fix that added unicode support to shlex as a side effect.