Delete preferences
When i go to about:config page and then " show all" , i see a lot of preferences have a trash bin at the end of the peference line. Is it safe to click on trash bin and delete all these preferences ?
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It's safe, but removing some of these preferences will switch corresponding modifications to defaults.
Note that a lot of these prefs are about storing states like when a specific feature has been used ('shown' prefs like the one for download in your screenshot) and a lot for timers for internal use (10 or 13 digit epoch timestamps).
You should always be cautious with modifying pref and make sure you understand what they are about and possibly check out the source code on the searchfox.org website.
Not sure if 3 is a valid value for save_converter_index (I'm aware of 0,1,2)
Are you possibly using a user.js file in the Firefox profile folder to initialize prefs on startup ?