I like to have separate Refresh/Stop and New Tab buttons in Firefox. I also use Tree-style Tabs. Here’s how to do it:

[Edit – note from May 1 2014 in context of FF29 – how to split refresh as a movable button again – https://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer/]

1) Open the customise toolbars dialog:

2) Look to the far right of your existing toolbars to see if you can find the two icons to drag over to new positions. The desired end result looks like:

Note – the New Tab button may look different in the customise toolbars dialog e.g. a green plus, but it displays as depicted – at least in current versions of Firefox (25 at present and counting ;-)).