Thank you for your question about the Firefox Sync service.

It appears from your question, that you are using Firefox Sync as a back up service. This is not what Sync is designed to do. The Firefox Sync service takes a copy of the data you wish to include and transfers it to a second device (typically a mobile device such as a tablet or a telephone) running a copy of Firefox. The storage in between all attached devices is both temporary and fragile and is not stable enough to be (and is not designed as) a reliable backup service.

If you have a copy of your Firefox profile for desktop Firefox, you may be able to recover your bookmarks and other data. Please have a read of this article.

If you have a copy of your data on a mobile version of Firefox, please follow these instructions to connect it to Firefox Sync to copy your data.

MrPikachu said

I have used my firefox account to sync bookmarks, but today after i reinstalled window, it did not work for me. As usual, all bookmarks appeared after i loged in, but not anymore, how i can find that backup file in firefox account ?

Please explain the statement that I bolded.

Did your bookmarks appear in Firefox after re-installing Windows and Firefox, but disappeared when you re-connected to Sync?

Did you change or reset the Firefox Account / Sync Password before re-connecting to Sync?

Thank you guys to notice my problem, i understand it and know how firefox sync working, i just re-install window for my pc and firmware for my mobilephone in the time, so i lost a backup file from my phone. I thought firefox sync worked as cloud like Google does it in Chrome. Thank anyway.