Hello sir jscher2000 and thank you for your help. interval set was on 15000, so it can't be issue for Firefox...
Today, I moved left-right number of tabs to test Firefox. I closed some tabs to see if reducing the extra tabs would fix this problem, Firefox changes 4-5 tabs and stops again...When I check from my Firefox account, that's always syncing...Even I click on (Sync now) to update the tabs but Firefox does not update and that problem was not fixed.