There's an easy way to backup your Firefox extensions: Firefox Extension Backup Extension. I know it has a funny name, so you can just call it FEBE. You can backup your extensions, your themes, your bookmarks and settings to a folder. Copy that folder to a CD, send it by email in an archive or just put it on an USB flash drive.
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How to restore extensions?
* Open Tools/Extensions from the menu
* Drag and drop the extensions from the backup folder to the extension dialog.
You can also restore extensions one by one from Tools/FEBE/Restore Extensions.
But what about the settings for each extension? If you backup your Firefox settings, you'll also have them.
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