Do you want to clear Firefox browsing history after x number of days? There is an extension for that. Firefox by default doesn’t allow to clear browser history after some days, say for example 10 days, it allows to clear recent browsing history for the last hour, two hours, four hours and today and Everything. What if you want to clear month-old history? Fortunately, there is an extension available in AMO to serve your purpose, meet History Janitor, which automatically removes history older than you specified number of days.
All you need to do is download and install the extension, click on its Options and enter the number of days for it to purge the history when Firefox starts.