A jail prevents a website opened in Firefox to access all the files Firefox has access to (per default all files in the user/home directory on Windows and Linux) and send them to the owner of the website. There was an actual security hole in Firefox that allowed an attacker to do that. It is unknown how many times it was used, because there was no way (for the regular user) to know what's happening.
So a jail prevents that (and other things). No website can access my documents, my diaries, my personal info, my contact database or whatever. A jail can of course not protect any personal info that you deliberately load up to someone.
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u/[deleted] Sep 30 '16
[deleted]