This puts a little navigation bar down in the bottom right of the window, with buttons that correspond to the LINK REL tags in the web page, letting you do forward, back, up, & top. This is handy, because it means that (when people use these tags) the "next" link is always in the same place, regardless of who wrote the page or how it is scrolled. Mozilla had this built in (the "Site Navigation Bar") but it was removed from Firefox.
When you pop up a menu over an image and select "Properties", this extension shows you any EXIF data in that image: shutter speed, etc.
This adds a new tab to the "Tools / Page Info" dialog that lists all of the named anchors in the current document.
Moving the Search field:
Not really an "extension", but it took me a while to notice that it's possible to move the "Web search" field up into the menubar area, instead of having it to the right of the URL field. This is nice because it gives you more space to read long URLs. You do this by clicking right on the menubar and selecting "Customize". The UI is pretty clunky, but it lets you get rid of much useless junk and move just about everything around.
Other search engines:
You can change the set of search engines on the menu to the right of the field by selecting "Customize", but that doesn't work very well (you have to be the owner of the /usr/lib/firefox-*/searchplugins/ directory, I think.) I deleted everything in that directory but Google, and added Webster, IMDB, and the Wayback Machine. Annoyingly, you have to re-do this every time you upgrade Firefox.