Hmm, is it just very slow or is the data really gone? You can check what data is in the Sync cloud for your Mozilla Account using the About Sync add-on:

https://addons.mozilla.org/firefox/addon/about-sync/

After you install the add-on, you can type or paste about:sync in the address bar and press Enter to load it. The page shows each category of data currently found in the Sync cloud.

Anything useful under bookmarks?

I'm not sure of the exact method you used to roll back your computer. In some scenarios, Windows saves your data in a temporary folder named

C:\Windows.old

If you do see that folder:

Set Windows to show hidden files and folders -- this Microsoft article has the steps:

https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-97fbc472-c603-9d90-91d0-1166d1d9f4b5

Then explore that folder up to this point (if it exists):

C:\Windows.old\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\

If you find recent data there, please pause and we can suggest how to restore old bookmark backups.

Hello, thank you very much for the tip. I tried the Windows.old folder method and there I managed to find a backup of my bookmarks from the beginning of the week. For me, problem solved. The only thing that surprises me is Firefox's approach to synchronization - I also use Chrome and Edge, and after reinstalling I got everything I had before reinstalling the operating system. Anyway, problem solved. Thanks again for the tip. Kind regards

I've also looked into AboutSync and in the bookmars section I have "274 records". Hovewer, that's after I looked through the Windows.old folder, so it might be bookmarks I've just loaded.

I don't know enough about Sync to know why it doesn't always give consistent results. But it's great news that you found and were able to restore a backup.

C:\Windows.old apparently erases itself after 28 days, so you may want to back up key folders from there to somewhere you control, like the Downloads or Documents folder.

I have no problem with the remaining files - I had a backup via OneDrive and Google Drive. I was simply surprised, because it turns out that Firefox "saw" the new installation, without tabs, and decided that this was the desired state. As I wrote, Chrome and Edge worked fine, only in this case it ended like it did. Thank you once again - problem solved. Kind regards