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Multiple questions on profile backups

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  • Nzaghachi ikpeazụ nke cor-el

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I will be moving all of my data to a new laptop and want to know if I need to have both my personal Firefox profile and the "default-release" profile backed up. The "default" one is over 3GB and has over 20k files in it, and my personal one is only 113 MB with 1300 files — and my thumb drive needs all the extra space it can get. (Why would the "default-release" profile be so much larger than my personal one?)

The old laptop runs on Windows and the new one will have the Linux OS. Will that make a difference in using a backed up Firefox profile? I mean, will the data still work in the new OS?

I will be moving all of my data to a new laptop and want to know if I need to have both my personal Firefox profile and the "default-release" profile backed up. The "default" one is over 3GB and has over 20k files in it, and my personal one is only 113 MB with 1300 files — and my thumb drive needs all the extra space it can get. (Why would the "default-release" profile be so much larger than my personal one?) The old laptop runs on Windows and the new one will have the Linux OS. Will that make a difference in using a backed up Firefox profile? I mean, will the data still work in the new OS?

All Replies (5)

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Which of the two files is the most recently used profiles and has your personal data?

Firefox uses two locations in the hidden "AppData" location for the Firefox profile folder. Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

  • C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).


You can copy certain files with Firefox closed to backup your personal data. Note that best is to avoid restoring a full profile folder and only restore important files that are safe to restore/transfer.


  • bookmarks and history: places.sqlite
  • favicons: favicons.sqlite
  • bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
  • cookies.sqlite for the Cookies
  • formhistory.sqlite for saved autocomplete Form Data
  • logins.json (encrypted logins) and key4.db (decryption key and primary password) for logins saved in the Password Manager
  • cert9.db for certificates stored in the Certificate Manager
  • persdict.dat for words added to the spell checker dictionary
  • permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
  • sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)
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How would I know which profile is most recently used? I'm the only person who uses this computer/Firefox, but the "default-release" profile folder is more than 26x larger than my personal profile folder. WHY?

You list a bunch of important files from the profile to back up. From which of the profile folders should I take these important files? And, perhaps most important of all, will those files copied from a Windows machine work on the Linux machine - yes or no?

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You can look at the date/time of the files in the profile folder to see what profile was last used. You can check the content of compatibility.ini in a text editor (do not double-click, but use "Open with") to see what Firefox version last used this profile. Yes, the files I listed above are data files and should work on Linux without problems.

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That is good to know. I would still like to know why the the "default-release" profile folder is more than 26x larger than my personal <user> profile folder.

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You can check what folders and files take this extra amount of space, could be about the storage folder.