I recently added Google Browser Sync to my growing list of extensions. I find it to be a very useful tool for working across multiple computers. To extend it’s usefulness even more, I have added it to the Portable Firefox I carry on a USB Memory Stick so I can have my firefox setting on any computer I use. You can get Google Browser Sync here. There are some drawbacks. It takes much longer to start and shutdown firefox. If you have to use it over dialup, it could be quite painful. If you do not trust Google with your information, this extension is not for you. This is because you are saving many of your Firefox settings, including saved passwords, on Google servers.
Overall, I think this is one of the most useful extensions I have and use everyday.