I really love the new Firefox3 release. I've had beta version before but just updated and all my add-ons are also updated and are working nicely. The firefox Add-ons page can be found here. I previously managed to seriously confuse my bookmarks by upgrading and then downgrading to Firefox2. Don't try this.
I have also now swapped to using del.icio.us the social bookmarking site. This has really nice integration with Firefox3, adding buttons to quickly bookmark a site. The best thing for me is that the left panel where bookmarks are typically listed in a browser can now toggle to display del.icio.us bookmarks complete with a searchable listing of tags. It is really slick. Very quick, well designed and much much more useful than tradiational bookmarks. I had got to the stage with Firefox2 where I was Google searching for sites that I knew were in my bookmarks because it would be faster to find them that way! The tag-based approach to data is so powerful I just groan when I see programmes not supporting tags.
Also in my add-ons is Zotero. This is an extension to help collect, manage and use bibliographic information. I'm an academic and spend a lot of time collecting journal articles. Zotero deserves a post by itself, soon I hope.
Lastly is Google Notebook. I don't really use this as a notebook for manually typing information (I use Journler). But Google Notebook has a great web-clipping function. OK this is also found in other software too but Google's implementation is pretty fool-proof.