The Firefox CPU problem was still causing me some problems but I seem to have fixed it... Touch wood...
After much futile poking around the web, The Mozilla support for firefox suggested that I remove and then re-install the latest release and try it with a different profile.
I did it the Gung-ho manner (on Windows 2000) by uninstalling the Firefix v1.5 using the \Control Panel\Add remove programs\ and then copying the bookmark file* (bookmarks.html) to the desktop. I then deleted the mozilla folder (beware doing this if you use Netscape Mail!). I re-installed Firefox 1.5 (13-12-05 at around midnight GMT) and copied the bookmarks.html back to its new home.
That seemed to do the trick.
* C:\Documents and Settings\--YOUR USER HERE--\Application Data\Mozilla\Firefox\Profiles\--SOME PROFILE HERE--\
This is how my Task Manager performance tab looked as I was busy re-installing the Google bar (or a variant of it that works with v 1.5) and ReminderFox...