I have been such a long time Firefox users since primitive ages of Firefox. I really liked how fast it was at the time, but so far it is not that fast anymore. It, however, is still the very first choice of me and most people because of how productivity it is. It's like why Palm OS was great--a lot of flavors to add on. Foxmarks, adblock plus, grab and drag, and so on are such a nice addition to Firefox, but most folks is not realized how handy Firefox sidebar is, including me until I discovered the use of it.
Basically, it is just a narrow browser. (or another tab if you like) It's better than just another tab because it won't disappear: it will stick on the side at all time. The question is why we use such a narrow place while we have such a wider one already. Yes, we will not find sidebar awesome than any regular tab or anything; It's just good for fast scanning some info or keeping watching something like constantly change w/o sacrifice other tasks. It's easier to try out and see if this works for you and of course I have some to recommend:-
1. Todo list from GMail – This is the one I use most; very simple todo list. I believe that you have to enable “Tasks” in GMail Settings | Lab first then add http://mail.google.com/tasks/ig?pli=1 in the bookmark and surely don’t forget to set it to load in the sidebar.
2. Google Calendar: – show what is on your calendar – the gadget on still faces technical problem; if you like to do the hard way, here it is on LifeHacker.com. It’s just read-only calendar – agenda so far.
3. Weather – pretty clean interface from Weather Channel.
http://xhtml.weather.com/xhtml/hbhf/03811 << This is a link; all you have to do is replaced the zipcode; then repeat the same step to get it work in sidebar.
4. Twitter – there is an extension taking care of this “Twitbin” – everything is automatic, but there is only one drawback, sidebar width will not be able to shrink narrower than 250px anymore. It might not be a problem with most people. Someone, however, like me who still like 4:3 screen will lose considerably a lot horizontal space.
6. Google Reader – This one is a bit awkward fitting in the sidebar; iPhone version works reasonably good although it’s too wide & uses too big font. You’ve got to try yourself. Actually there is PDA version, but I don’t think it would suit in here. http://www.google.com/reader/i/ << here you go.
You will see that most of the sites suited using in here is PDA/Phone version. Since they all are designed with the limited screen, esp. width, in mind. Putting their link in bookmark bar, check "load in the sidebar." You'll be all set.
Happy using what you like and get the most of it. This probably is the another reason why I couldn't switch to ie, chrome, safari, or anything else soon. If you find anything worth for you, please share since it might be worth for someone else too =)