Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

This is your current profile. Copy the folder As Is to a folder on a thumb drive. Then Copy the folder to the new system.

Hi FredMcD. You're a robot, right? You've given me the old-fashioned solution. The file structure on Windows 10 is obviously different. Plus, I seem to have 3 different profiles in my Mozilla folder. I tried the biggest one and that gave me an old profile.

It is best to avoid copying the full profile folder and only restore personal data you really need. Extensions are best installed as well to make sure they recognize your current OS.

You can copy files like these to the current Firefoxx profile folder to recover specific data. Be cautious with copying SQLite databases if you previously had problems.

• bookmarks and history: places.sqlite
• bookmark backups: JSON backups in the bookmarkbackups folder
• cookies.sqlite for the cookies
• formhistory.sqlite for saved form data
• logins.json and key3.db (decryption key) for Passwords saved in the Password Manager
• cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences
• sessionstore.js for open tabs and pinned tabs

Firefox uses two locations for the Firefox profile folder, so make sure to look in the correct location. Location used for the main profile in "AppData\Roaming" that keeps your personal data.

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

Location used for the disk cache and other temporary files in "AppData\Local".

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

See also:

• https://support.mozilla.org/en-US/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Do you have any insights about my 3 original profiles and how there's supposed to squeeze into one?

It is not possible to merge data from various profiles apart from using Sync or possibly FEBE if you used this extension before.

What kind of data would you like to merge? Are that your bookmarks?

Merging bookmarks is only possible by importing an HTML backup. Restoring a JSON backup (bookmarkbackups folder) will always replace all bookmarks. You would have to create an HTML backup before restoring the next JSON backup. Note that duplicates aren't handled and you would have to do this manually yourself.

Whenever I used Firefox before, I only had one identity and never had to select a different persona. Yet I have 3 different profile folders to select from. So I don't know how to continue.

Choose the profile whose folder has the most recent date.