When you ask Firefox to open a new tab, Firefox displays a list of top websites of your own history. This list consists of 8 recommended websites placed in one row and named Top Sites.
Their choice is based on your history and they are the sites you visit the most. You have the option to delete a selection of it Firefox but the next option of the browser automatically takes its place. You also have the ability to discard sites and effectively hide them from the Top Sites entry, as well as rearrange the top locations using drag and drop.
Since Top Sites are quite easy and convenient, let's see how you can increase the number of Firefox suggestions and a series of 8 tiles to make more rows.
First, it's easy to increase the number of rows to two, as Mozilla has added an option for this job. Clicking the gear icon on a "New Tab" (top right) shows various options. Find among them the "show two rows" option in the "Top Sites" area, mark it and a second row will be added to the "Top Sites" when you request a new tab.
However, if the two rows of top websites are not enough for you and you want more rows, Firefox gives you the option and you should take the following steps:
1. Open a new tab and copy-paste it to this address bar:
about: config? filter = browser.newtabpage.activity-stream.topSitesRows
2. On the page that appears, say "Take the risk", then double-click browser.newtabpage.activity-stream.topSitesRows and change the existing value to the number with the desired number of rows.
3. Click ok.
Now check out a new tab and you will see the change immediately and, if necessary, modify the number of lines according to your wishes. See below how our selection appears with 4 rows: