Are you possibly using Sync the sync bookmarks because that could be a cause? Also some problems with the places.sqlite database can cause Firefox to restore bookmarks automatically from a JSON backup to rebuild places.sqlite. You can check for problems with the <b>places.sqlite</b> database (bookmarks and history) in the Firefox profile folder. *https://addons.mozilla.org/firefox/addon/places-maintenance/ See also: *http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox *https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

Thanks a lot, cor-el. Yes, I'm using Sync. I feel it could be due to Sync as well. But do you think Sync should sync my latest modifications to the server or other devices right after I save preference or close Firefox? I still cannot understand why it reverts my manual changes. I manually deleted all the session files of my system and those of Firefox last night. And made my modifications to the bookmarks. After several restarts of Firefox, the changes kept. This morning, after I booted my system and started Firefox, I noticed that the changes are kept. But, after a while, they are reverted. So, it has a big chance to be caused by the Sync feature. Maybe there is any mechanism of Sync not being implemented correct. Or, is Sync designed to be like this? But this feature is too useful to be dumped. For your suggestions, I used the extension you suggested just now. It reports Firefox to be sane. Here is the statitics in case it helps: [http://pastebin.com/V98HwCzn Report of Places Maintenance] And for the articles you provided, # I can add new bookmarks, so I guess my Bookmarks File is writable. # The places.sqlite seems working well. Thanks again for your help. And do you have a workaround to make Sync not revert my modifications? '''========== Updated on 15:45 06/12/2016''' The disconnect and reconnect operations on secondary devices solved the problem. Today, the bookmarks are sync-ed to my mobile phone. After manually sync on my phone, then on the main device, the manually sorted bookmarks are kept. If anyone having the same problem wants a detailed procedure, please refer to the section below. '''========== Updated on 15:14 05/12/2016''' After previous investigation, based on cor-el's suggestion, I performed the following tweaks, and the bookmarks look good until now. And Sync must be the culprit. Operations: # Disconnect sync-ed devices (PC at work, mobile phone) except the main one of my laptop # Sort bookmarks manually on the main device which still connected to Sync server # Sync to server manually # Delete and create a new profile for Firefox on the PC at work following this <[[Use the Profile Manager to create and remove Firefox profiles|Use the Profile Manager to create and remove Firefox profiles]] > # Clear DATA of Firefox app on the mobile phone following this <[https://www.cnet.com/how-to/how-to-clear-app-cache-and-app-data-in-android-6-0-marshmallow/ How to clear app cache and app data in Android 6.0 Marshmallow] > # Re-connect other devices and sync again So far so good except bookmarks have not being sync-ed to my mobile phone after reconnected more than 5 hours. I still think the working scheme of Sync should be tweaked to make sure the very last active device has the priority to be the ultimate version of sync-ed information. I will update this post after bookmarks being sync-ed to my mobile phone.