1. Were you using a Firefox 29 or later version prior to "reinstalling everything"?

2. Did you have any problems signing in to Firefox Accounts? Like maybe you needed to request a new Password for your Sync / Firefox Accounts account?

1. I was using Ubuntu 14.04 LTS's firefox. So the version was at least 32. I haven't encounter any problem to sign in Sync.

2. May be I have requested a new password because i forgot it :$. But I eventually successfully connect my firefox to Sync.

2. Sorry, if you did request a new Password, that's why Sync is "empty".

The problem is that when you request new Password for your Sync account, your data is wiped from the Sync server to keep your data from falling into the wrong hands. There is a warning message stating that when you request a new Password.

The basic concept of Sync is that with syncing multiple devices, that in addition to the Sync server user data will be on at least one device in the event of one device getting lost, or one device stops working, or one device needs to be formatted or the drive needs to replaced. If you did re-installation of "both devices" (in your case, two operating systems on the same hard drive), you went beyond what Sync was intended to do.

Normally, that's not a big deal because Sync still has your data. But when requesting a new Password you effectively "formatted" your data away.

(- Do you have any other devices connected to that Sync account? -) If you do, do go online with that device. There may be hope that one has your data.

It was my only device :( I'm very disappointed by this parameter i have lost all my strategic foresight. Is there any way to correct my bad operation ?

If you were sure that you asked for a new Password for your Sync account, I would tell you up front that your data is gone and there's no way to recover it.

And it sounds like you are logged in to Sync and your data isn't getting to either OS / Firefox after signing in, so my guess is that your data is gone.

I can't think of any way to recover from that, short of running Recuva to see what data may be located for restoration. But that is a long shot for two operating systems. https://www.piriform.com/recuva Partial files getting recovered won't hack it for most if not all Profile files.

I understand why you were using Sync between two versions of Firefox which are installed on different operating systems, but in that situation it would have been better to have an outside device on that Sync account or you had reinstalled your operating systems separately, at separate times - so that one still had your Firefox data. Or you had asked us first how to proceed and we would have recommended you backing up both Firefox Profiles before proceeding. I am assuming that you have only one drive in that PC - so that you couldn't do that easily especially if you wanted to change partition sizes, which have done a few times over the years with dual boot. Or your data was on a separate partition or separate hard drive, so that you could blow away and restore the drive with your programs and operating systems, while your data "is safe and warm" elsewhere. Sorry, I am starting to feel that I am "beating you up" over this loss of yours and I don't come across as sympathetic. Like woulda and shoulda would help you now.

My harddrive decided that this life was long enough for it so I had to replace it. New OS, new shiny Firefox... I did write my password down when I created, but somehow it didn't work, so I just requested a new one as one man do in the 21th century. Of chourse I couldn't syncronize my precious bookmarks (I don't care about passwords too much, I can request a new one even on the crappiest websites) so I just read the troubleshoots then the forums and now, I found my answer. According to this, I should have known that my HDD was going to break down and should have bought another e.g. computer (maybe with another desk too) to keep it sycronized. It has to be another computer because I couldn't syncronize firefox with my tablet no matter how hard I tried. This means that while I'm working on my computer my unexisting computer should be running too next to me doing almost nothing just becasuse of the Firefox Sync. Ok, I understand everything and I have only one question left.

ARE YOU JOKING WITH ME???

What's the point of syncronization according to mozilla? (Ok, maybe two questions...)