Expiration happens on idle, but it's not instant.
First expiration step may take some time and blocks UI during that time, next ones will be much smaller and should not create any issue.

Warning: the Places expiration system works to provide you the best performances in an automatic way. Using this add-on to override its functionality, may create unexpected performance problems and is not needed. Thus, unless you have clear reasons to need the functionality exposed by this add-on, you should not use it.

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• License: Mozilla Public License 1.1
• Weekly downloads: 5
• Average daily users: 1179
• Rating: 4.2727/5 of 22 ratings
• Created: 2011-07-28 16:48:22
• Last updated: 2017-06-20 14:15:15
• Homepage:
• Support site and email
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• Orig: https://addons.mozilla.org/en-US/firefox/addon/expire-history-by-days/
• API: expire-history-by-days@bonardo.net