The latest Firefox 5 update inadvertently broke the Google Toolbar. Frankly the version 5 came too fast in the wake of Firefox 4 which broke the Delicious bookmarking plugin.
Since I have several PC's, looks like i'm not going to update the rest of my PC's to Firefox 5 until Google comes up with an updated plugin.
--- [update 27/6/2011]
Found this thread in Mozilla support. By using this compatibility reporter extension, you could make Google Toolbar to work with Firefox 5. I gave it a try and it works! It also made my Trend Micro toolbar working in Firefox 5 too! Nice fix!
And finally I gave the Delicious toolbar a go too, and voila! It works! No need to wait for update from Delicious. Anyway, with the breaking of Delicious I found a better bookmarking service from Xmarks.com. Since I still got many bookmarks in Delicious, I guess I will be keeping both service for now.
The Google Toolbar 7.1.20110512W version works in Firefox 5.0 by using the Compatibility Reporter extension.