Dozens of open tabs lost! Synced them, reinstalled Windows and Firefox, set up Sync, looked up "tabs on other devices" in history - Only a third or so listed.

Question

Dozens of open tabs lost! Synced them, reinstalled Windows and Firefox, set up Sync, looked up "tabs on other devices" in history -> Only a third or so listed.
Usually, when I reinstall Windows, I manually backup my Firefox bookmarks and open tabs, but because Sync has worked perfectly for me in the past, I didn't do it this time. I backed up all my other files and settings and whatnot from the rest of my system, then in Firefox I hit the Sync button (put in on the GUI so I see if it's syncing or done) for the last time, waited until the green rotating dot was gone, closed Firefox, shut down the system, formatted the drive and reinstalled Windows and Firefox. After setting up Sync I waited a while to make sure everything is on my system.
Then I wanted to have all the tabs and windows open as I had on the old system, so I went to the history and "tabs on other devices". I could already see from the small size of the list, that tabs were missing. I went through all of them, opened them and sorted them into windows.
-> Maybe about two thirds of the tabs I had open on my old system are gone!
Not just new tabs that might not have been synced (although the button showed sync was complete), but also old tabs (from weeks ago) that must have been synced before. Actually maybe especially or even only the old ones are lost. And the lost tabs are not just from one of the open windows, but from several.
What can I do to recover my lost tabs?
They contain important websites that I didn't bookmark yet, articles that I wanted to read, windows with dozens of tabs on a topic that I did some research for and stuff like that.
What does the green circle mean? Does it indicate that the sync is complete (to the Firefox servers) or just that the changes have been saved locally and set up for some next scheduled sync with the Firefox servers?
Further info:
I always run the newest stable release of Firefox (v19 on both systems). Old system was Windows 8 Enterprise Evaluation, new system is Windows 8 Pro. My used Sync quota is about 10 MB. I had 4 Firefox windows open with different amount of tabs in each of them, from 10 to maybe 80 with an overall amount of 150-200. Most of them weren't loaded. Firefox start is set to "load last session" and tabs are set to "load when selected". Memory usage 500 MB - 1 GB.

Answer 1

I have most of the items bookmarked. Having 200 - 300 tabs open saves time, since I don't have to go to the bookmarks, if the tabs are already open. One problem has been crashes, but as long as I can restore a previous session, I can still get to the data. I had quite a few tabs open when I was planning to purchase a new computer. I also follow the stock market, and the weather. I was also looking at various VIrtual Private Networking (VPN) sites. At one time, I probably had 20 - 50 tabs open, just for those. I did bookmark a few, but unfortunately lost the VPN tabs, when Firefox crashed. Even if I don`t go to a particular group of tabs very often, leaving it open is faster, especially since I will know where they are. I often put the computer in standby mode, rather than shutting it off, especially if I will remain in the same location.