Hi Goetz! There are a couple of ways that I'd recommend doing this.

If you only have bookmarks to transfer, the easiest thing to do is to export your bookmarks on Windows XP to HTML and then import them to Windows 10. But that only works for bookmarks.

If you are looking to recover everything, you'd probably find it easiest to move the entire profile folder over from your Windows XP computer.

To do this, first it's best to install the same version of Firefox that you have on your Windows XP onto your Windows 10. All the past Firefox versions can be found on this website. Do not pick one that's labelled "ESR". When you pick the version on that website, you need to pick the operating system (probably win64 for you) and the language you want.

When you install it on Firefox, make sure that you do not check the option to open Firefox when you are done installing. We don't want to open it yet.

Now on your Windows XP computer, press the Windows key + R on your keyboard, type in %appdata%\Mozilla\Firefox\Profiles and press Ok. This is where your data is stored. Copy the folder (I assume you will have just 1) to a USB or some other device that can be used to transfer it to the Windows 10 computer.

Now on your Windows 10 computer, press the Windows key + R on your keyboard, type in %appdata%\Mozilla\Firefox\Profiles and press Ok. Copy the folder that you saved from your Windows XP computer to this location on your Windows 10 computer.

Next, assuming you have a Firefox desktop shortcut, right-click on it and select Properties. Add -P to the end of the Target text box, outside of the quotes with a space after the quote. Then press Ok.

When you open Firefox using that desktop shortcut, it will display the profile manager. You will want to press Create profile. Once you get to the page where you can set a folder name, press the Choose folder button. From the file browser, select the folder that you moved from Windows XP.

Once you've created the new profile, you can select it from the profile manager and open Firefox. When you open Firefox, it should have all of your data from Windows XP there as well. Then you can allow Firefox to update. That will help to make sure that your data is converted to the new profile structure.

Finally, you want to remove the profile manager from the Firefox desktop shortcut so that it's not opening each time that you start Firefox. Right-click on it and select Properties. Remove what you added above.

Hope this helps.

NOTE: As a side note, you will not be able to carry over any add-ons that you had installed on your Windows XP version of Firefox. This is because Firefox changed the way that add-ons work and the old add-ons don't work with the new version of Firefox. Fortunately, most add-ons have a new version available to download from the Firefox add-on website.

Note that best is not to copy a full profile folder because there are files that have absolute file paths and the Firefox 52 version for XP has a lot of files that have been replaced by newer version in Firefox 78, so you will end up with a profile that has a mixture of files.

You can copy specific files with Firefox closed to the current profile folder to transfer/recover personal data.

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite (55+)
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (passwords) and key4.db (58+) or key3.db (57 and older) (decryption key) for Passwords saved in the Password Manager
  if you only have key3.db then make sure to remove an existing key4.db
• cert9.db (58+) or cert8.db (57 and older) for (intermediate) certificates stored in the Certificate Manager
  if you only have cert8.db then make sure to remove an existing cert9.db
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)

On XP you have sessionstore.js for your tabs, so you will have to remove a possible sessionstore.jsonlz4 file and the sessionstore-backups folder in the Firefox 78 profile if you want to transfer session data.