umm, this is like the third iteration of it... (now v0.5b) they released this months ago for FF compatibility. They didn't copy/follow Google's ideas or anything.
I've used Yahoo Companion (the Yahoo toolbar) since IE5 was released. I didn't switch to FF for a while because Y! Companion didn't support it. Mozdev came out with their own version and I jumped on it. This Y! version is not any better than Mozdev's so far, but whatever.
Anyway, the reason I do use the Yahoo toolbar is because I use:
Yahoo Mail
Yahoo Calendar
My Yahoo
Yahoo Bookmarks (most convenient by far)
Yahoo Briefcase
It also allows you to create custom buttons and has a popup blocker (before FF even came out). So why not use it if I'm already using all that ?