Are you referring to the new tab page?

Yes Sir/Madam, the New Tab Page where I now have 3 rows of 4 icons or certain pages of the most used or most frequented webpage. Can this be increased to 16 ?

Thank You.

Page Zoom will affect the number of rows & columns that are displayed. https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

Or you can modify these 2 hidden prefs by using the config editor. browser.newtabpage.rows browser.newtabpage.columns

https://support.mozilla.org/en-US/kb/about-config-editor-firefox