If I had to pick only one plugin for firefox it would be the Web Developer Toolbar add on. I use it every day. I’m not really a developer. I do make simple web pages but I use it most the time evaluating websites. I use it to look at a site when I get an email or phone call from a prospect. I also use it when I’m link building.
The next thing that I do is click on the information tab and go to “view meta tag information”. I can tell if they are using meta tags at all. Sometimes you see all kinds of odd tags like “revisit after” which no search engine uses. Sometimes I see they have 2 descriptions.
I like to use the resize feature. Click on the resize pull down menu and change the size of the window to see how the site looks in different sizes.
This is not for SEO but I like to use the “Display Anchors” feature in the information pull down menu. I use this when I want to link to a specific forum post.
Under Images you can click on “display alt attributes” to see if they are using them. You can also tell it to show you images size. Find broken images is helpful too. “Show images full size” will show you real quick if they are not optimizing their images.
The Web Developer toolbar does quite a bit more but since I’m not a developer I don’t really use it. Let me know what your favorite features are in the comments. I would love to hear them.