Here is a dump of my current FireFox add-ons.
Some of them are particularly relevant to SEO/SEM:
- Diigo -- Great when you have a group that wants to annotate web pages to share notes on SEO mods you want to do for a page.
- Live PageRank -- Get the PR for a page.
- Send Tab URLs -- A quick and dirty way to get a dump of page URLs and descriptions by opening up the pages you are interested in Tabs, then selecting this option in the File menu.