Afaik, CCleaner does let you choose to install (or not) the Yahoo toolbar. Maybe it was because you were doing an update and it knew not to install it? Or did it install the toolbar?
My advice:just use the slim or portable CCleaner. No yahoo toolbar included.
In that case, I would redownload Firefox portable, not tweak anything or add anything, and see if its still the same.
Alternatively, you could do an actual install of Firefox 3 and see if it still happens.
I too prefer the old UI. Also I noticed the drive tab can have a lot of empty space:
It needs to be changed to use up the space. Maybe put "Properties" underneath "Status" and then move the file list to the right of those two?