How to import passwords from Firefox 3 to the new Firefox 4?
3 Answers
In case you were using Firefox Sync (formerly know as "Mozilla Sync") on 3.x, then your passwords will be synced automatically as soon as you log on to it.
Starting from Firefox 4.x, the Firefox Sync is a built in, including all the future versions.
You should be able to export your Firefox 3.x passwords in the Security tab shown when you select the Tools | Options... menu item. I don't have Firefox 4 installed, but there should be a similar spot in its Options to import them.
Update
Sorry I didn't realize this wasn't core functionality. In order to export/import your passwords you'll need to install the password-exporter Firefox addon. It supports Firefox 3.0 - 7.*
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If the old Firefox was at least version 3.5 and you did not set a master password you can simply copy the file signons.sqlite
from your old profile to the new profile directory.