So Firefox 2.0 is out and can be downloaded ... yay!
but upgrading the home Fedora box was an unnecessarily geeky activity.
I don't think I missed the obvious but this was how it went...

Go to Mozilla
find the right distro (this was firefox-2.0.0.4.tar.gz )
and download.
As root (if you do it as a user, you don't change things for *all* users.)

    tar -xzvf firefox-2.0.0.4.tar.gz 

in the resulting directory (blandly named "firefox") locate the firefox script
and replace the current 1.5 version script.

    cp  firefox/firefox /usr/bin/firefox

Now, this script is looking for (as a default directory) /usr/local/lib/firefox-2.0.0.4.
Two minor point here ...
1. the 1.5 distro on fedora is in /usr/lib (not /usr/local/lib)
2. The directory you have just extracted has to be named as the version
so ... move & rename the whole thing.

    mv firefox /usr/local/lib/firefox-2.0.0.4

Now click on your current desktop icon & it should fire up the new version 2.0
(which on startup suggests, wrongly, that Firefox is not the default browser ..etc ...)

Now I would have thought that there would have been some sort of installation advice,
but the Mozilla page has a *single* paragraph on installing:

Please note that installing Firefox 2 will overwrite your existing installation of Firefox. You won't lose any of your bookmarks or browsing history, but some of your extensions and other add-ons might not work until updates for them are made available.

... NOW THAT'S REAL HELPFUL!
I would have thought that by now this would have been easier!