Now that Google Notebook is no longer actively developed, Google decided to hide the link to the Firefox extension and to remove the Notebook support from Google Toolbar. It's probably a wise decision, since new users can't sign up for Google Notebook and Google won't be able to make sure that the add-ons will be compatible with the future versions of Firefox and Internet Explorer.
The only alternative to the Notebook add-ons is a bookmarklet, but it's too basic and the confirmation message is annoying. Google Notebook's integration with the browser is what made the service very powerful: you could copy the text from a web page in a notebook with one click.
I don't know for how long you'll still be able to download the Firefox extension from Google, but here's the download link:
If you use a beta version of Firefox 3.5, here's a way to install the extension:
* save the extension to your computer: right-click and select "save link as".
* download 7-zip, an excellent open-source software for archiving files, and install it.
* right-click on the extension (google-notebook.xpi), click on "7-zip" and then on "open archive". Right-click on "install.rdf", select "Edit" and replace "3.0.*" with "4.0.*". Save the file, close Notepad and update the archive.
* now you can drag the .xpi file to your browser and install the extension.
Internet Explorer users that want to download the Google Notebook add-on can try this link:
6 minutes ago