Had you been using Sync before that Win10 upgrade, connecting your new Firefox installation to to your Sync Account / Firefox Account would have made the transition quite easy to that "new device" with Win10.

Beyond that, the smart thing to do would have been to backup your Firefox Profile before you upgraded to Windows 10, as you also should have done with the user data for all the other applications on that device. Then you could have "restored" your Firefox user data to Firefox 70 with no (to little) loss of your Firefox user data.

What other devices are you using and have Firefox installed? A Desktop / Laptop device would be preferable, but a "Mobile" device running Firefox for Android or Firefox for iOS would have most of your user data (with a smaller amount of "history").

Thank you The Edmeister. I don't think I have synced my firefox account before upgrading to win10- I thought it would automatically saved all the bookmarks.

Unfortunately, I only use firefox with this lenovo thinkpad. Does it mean all my bookmarks are nowhere to find?

(if the answer is yes, it is quite a pity; yet, I'll take it as a new chance to "renew" FF!)

Thanks again!!

You were either using Sync, or you weren't using Sync. It is an optional service that requires a purposeful sign up process and needs to continue to be used to be effective; not a one time backup service and then turn it off until you think you may need it - it is not a "cloud" service.

Firefox does automatically backup the users bookmarks and it saves 15 backup files before overwriting the older backups - but those backups are saved on the users device, not saved to anywhere online unless the user uses Sync or some other type of similar "sync" service or a "cloud" service.

That said, when a new Windows OS is installed the new operating system typically saves the users existing data to a \windows.old folder on the C:\ drive. Perhaps your Firefox data files are in that windows.old folder; look for a places.sqlite file as a start.