After update Mozilla Firefox to 3.5,I can’t install google toolbar by clicking on Download google toolbar 5 in toolbar.google.com.And following is how to manually install google toolbar.
First,download the toolbar and save to your desktop:
Windows

Mac

Linux

Click Allow if you see this window

google_toolbar

Then ,click and drag the Toolbar file from your desktop into a Firefox window.

And a Software install window appear.Click install Now.

Restart Firefox.