Contains all your extensions and bookmarks and userchrome for the default user.
The bookmarks.html you can just directly copy from one machine to the other. Works fine, done it a few times. Between the three machines I like to keep the bookmarks synched up in, I just e-mail myself a copy of the bookmarks file whenever I make any significant changes, and then make sure to download it to the other two copies of FF.
As for the extensions, I'm not 100% on direct copy. For safety, I'd just make a list and manually install all the extensions
I know it all sounds kinda manual, but it only takes a minute at the end of the day (or whenever) to upload the bookmarks file. As for extensions, you only have to install them once, then just have the browser auto-update them on both machines. This should keep things pretty synched up.