As I recall you can save your Extensions and Themes in Firefox by removing Firefox from your Add/Remove Programs and nowhere else (do not delete any other files or folders).

If you're upgrading to a new version, however, your Extensions and Themes "..... will be disabled until Firefox determines that either a) they are compatible with the new release or b) there are newer versions available that are compatible. This is because Firefox changes from release to release and as such the ways in which some extensions integrate changes, meaning there can be problems when running an older extension with a newer version of Firefox. "

http://www.mozilla.org/products/firefox/releases/#new