firefox 58 does not have new addons to customize tab position below address bar, this simple userChrome.css file can be created to achieve it.
#cd /home/pawan/.mozilla/firefox/bl6ofr4t.carmatec/chrome
here create a directory chrome and file userChrome.css then add below lines, restart firefox you should have tabs below address bar now.
/* Tab bar below Navigation & Bookmarks Toolbars
For best results, show Title Bar or Menu Bar */
#nav-bar { /* main toolbar */
-moz-box-ordinal-group: 1 !important;
border-top-width: 0 !important;
}
#PersonalToolbar { /* bookmarks toolbar */
-moz-box-ordinal-group: 2 !important;
}
#TabsToolbar { /* tab bar */
-moz-box-ordinal-group: 3 !important;
}

Increasing number of top sites on firefox. change value of browser.newtabpage.activity-stream.topSitesCount from 8 to 36

Related Articles
How We Set Up Our KVM Hypervisor: From Bare Metal to Production-Ready VM Host
Detailed walkthrough of building a dedicated KVM/libvirt hypervisor with XFS tuning, hugepages, 10GbE tuning, and automation.
Building a Predictable KVM Infrastructure: From Chaos to Control
How to engineer a predictable KVM-based infrastructure focusing on repeatability, observability, and operational safety.
Virt-sparsify
How to use virt-sparsify to reclaim unused disk space from qcow2 VM images.