A nice firefox add-on i have so i can immediately judge how much traffic a website has and how important it is, is search status. I got so used to it I find it hard to browse without it.
From the description:

Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more.

Another good seo add-on got really used to is flagfox. It shows the country the server hosting the site is. This can usually help you figure out for which country the site is optimized for, (unless things like bad seo, trust or processing power/bandwidth come into play).
This add-on shows a little flag for the country the server is in on the status bar.
From the description of the add-on:

Displays a country flag depicting the location of the current website's server and provides quick access to detailed location and web server information.

A third add-on i used for a while, but stoped doing so cause it was way too intrusive is seo quake. It's "keyword density analysis" was rather cool.
From the description of the add-on:

Seoquake is a Mozilla Firefox SEO extension aimed primarily at helping web masters who deal with search engine optimization(SEO) and internet promotion of web sites. Seoquake allows to obtain and investigate many important SEO parameters of the internet...