IE Tab
Let’s you open a tab running IE. Great for checking your site in multiple browsers quickly.
https://addons.mozilla.org/firefox/1419/

Session Saver
Remember’s what pages you are on in case your browser crashes or closes unexpectedly. Automatically re-opens them all when you re-start firefox.
https://addons.mozilla.org/firefox/436/

Disable Targets for Downloads
Let’s you avoid that annoying empty tab that pops up every time you download something. Helps eliminate the irritation.
https://addons.mozilla.org/addon.php?id=241

Hope that helps out as well. Some other cool ones include CSS Viewer and Sage.

a[rel~="nofollow"] {
border: thin dashed firebrick ! important;
background-color: rgb(255, 200, 200) ! important;
}

to userContent.css in the chrome directory to get highlighted nofollows.