Firefox Environment Backup Extension ( FEBE ) is another useful extension which backs up your extensions, themes, and (optionally) your bookmarks, preferences, passwords, cookies and just about everything else Firefox offers including your entire profile. This is pretty useful in case you want to synchronize between your home and office computer and laptops.

It also allows:

• User defined backup where you can choose what you want to and not want to backup.
• Scheduled Backup makes it easy.

It will actually rebuild your extensions individually into installable .xpi files which you can just take anywhere and Install it again. There is no need to download again from internet in case you are re installing your PC or moving to a new one.

It has even some debug options which you can choose to find if it goes wrong. Overall I think its a must have utility and a good idea to backup every time. Download the plugin here