I used speeddials for some years, then some other obscure addons and local html files. A few years ago, via G-Hacks, I found Humble New Tab Page[0], and it's been love at first sight. It has some little quirks if you are clumsy as I am and move a folder to another column, but it's not that hard to undo by hand. As a plus, it was a good excuse to bookmark frequent pages and put them inside folders.

[0] https://addons.mozilla.org/en-US/firefox/addon/humble-new-ta...