Toolbox Plus allows you to customize the mode (icons and text/icons/text), icon size (small/large), flexibility (modifies the toolbar buttons to occupy the entire toolbar), and text position (below icon or beside icon) of each individual toolbar, similar to SeaMonkey's toolbar functionality.
Simply download the contents of the "src" folder and pack the contents into a .zip file. Then, rename the file to .xpi and drag into the browser.
Check the releases page of this repository.