I have been using the iGoogle sidebar extension for Firefox, but it is not working for FF3. However, I discovered it is fairly easy to replicate the effect by simply creating a bookmark for iGoogle and having it open in the sidebar. It just needs a few style tweaks (based on my Clean iGoogle - hide what you don't use) to make it work right.

It basically just eliminates the whitespace and removes the unnecessary components (header, footer, tabs), and floats the columns so they occur in one long column in the sidebar instead of side-by-side. I've also made another version, iGoogle sidebar display in Firefox - with tabs, if you want to leave the tabs visible.