I have a bunch of Firefox extensions. But in case if I have to reinstall my computer, how do I restore them all? Sort of like Dropbox syncing all my files.
There are couple of ways.
Depending on your exact needs, either of the above two schemes will work.
Note that the FEBE sequence can actually backup a lot more than just your extension list.
The Collections method meanwhile is very good to refer your extension list to someone else, or
https://addons.mozilla.org/en-US/firefox/addon/11778 you might want to try this.
To synchronize your extension, you just synchronize your firefox profile. Use dropbox to synchronize you Firefox profile folder.
Typically I don't synchronize my extensions, this is how I maintain my data consistent across all computers
On Current System 1. Bookmark all my extensions on Delicious 2. I use Foxmarks (now XMarks) to synchronize passwords and bookmarks
Goto second system
This way I keep my data consistent across systems, and I still keep each installation clean (by not copying extensions and profile)
Alternative to Xmarks you may also use Mozilla Weave.