I have to thank Galen for repeatedly telling me to try Piclens - it's by far the coolest Firefox extension ever designed (and yes I know I'm in a small minority that think any are cool). If you ever look through flickr albums, search google images, or browse youtube, installing Piclens will completely change your experience. Instead of using whatever interface those sights has, Piclens allows you to zoom in and out seamlessly through all of the images/videos available in a slick interface that is easy and entertaining to use. It actually can't be explained without seeing or trying it, but if you use Firefox you should be taking the suggestion and trying this extension right now. If you head over to the product page and watch some demo videos that can't really do it justice.
Another Firefox extension that I use often is ScribeFire, a blogging platform that builds right into your browser. It was helpful but buggy in the beginning; now it has gotten a million times stronger in recent months with support for blogger categories, much more customization, integrated image search and formatting, among other things. If you blog via a web-based blog tool like blogger or wordpress.com, it's probably going to make your life a lot easier.