I have been using Firefox for a long time now. I almost never load Internet Explorer but I dallied with the Opera browser for a while, and although it is a faster loader than Firefox I find I return to Firefox every time partly because of the great extensions (mini programs) it offers. I know Opera has them also and I have downloaded some but I think Firefox's selection is better.
Here are some of my favourites:
Adblock, gets rid of all those pesky advertisments that slow down your computer
Thumbstrips which takes a picture of each site you visit so it is easy to flip back and fore to find the page you were at a moment ao. It is a visual history and much better than clicking on history I think. Plus you can save or share a particular search history if you wish.
Tab Catalog which is similar but it shows differently, I downloaded it to see which one I prefer and I think Thumbstrips is the winner.
Colourful Tabs which is helpful because each tab is a different colour and makes them easy to distinguish.
Session Manager which returns you to what you were doing when you are bounced off the internet of your computer crashes.
Split Panel which is great because you can see two different sites at once and drag and drop between. I used to load my browser twice to do this now I don't have to.
Hyperword makes every word on the web interactive. Searches, translates, references, maps, email etc. Very good.
Jump Knowledge Web Page Annotator I am not sure I will use this one but it enables you to add notes to any web page.
To find out more visit Firefox Extensions
Also I have been spending some time at BlogCatalog and MyBlogLog and there is a new site called Peopleized which is designed for people to interview each other.