In order to do this you have to log them into the same sync account. So if they are currently signed in to two different accounts (that's what it sounds like):

1. Go to the sync preference panel on the secondary profile and "Unlink This Device."
2. Then set up sync and say you already have an account.
3. You'll get a 12-digit code that is good for a few minutes. Write it down.
4. Start back up with the main profile.
5. Open the sync Options panel and "Pair a device"
6. Enter the 12-digit code you wrote down.

Now when you start up again with secondary profile it will sync with the main one.

Hi,

Sorry if I was unclear. I meant that I'm using same sync profile in both Firefox profiles. The sync is working correctly between those two Firefox profiles apart from tabs being not visible between Firefox profiles. So for example history data is visible on both profiles.

Br,

jjkola

Could it just be that the "computer" name in each Sync option panel is the same? Maybe try giving one a new name.

I did try that even before you suggested it but it didn't have any effect.

Hi!

I have a strong hypothesis. Your tabs don't have enough time to be synced.

Try to use "Sync now" in your Firefox Menu before closing the first profile and give it some time (half a minute should be enough if you don't have too many tabs) before closing the browser completely.

In a side note, what versions of Firefox are you running? If the solution above doesn't work, I will recommend you to open a bug with the log because it may be something related to your configuration. The logs will tell us that. The following links contains detailed instructions how to do it: https://philikon.wordpress.com/2011/06/13/how-to-file-a-good-sync-bug/