Open Customize and place a Flexible Space on each side of the Bookmarks Toolbar Items. Your bookmarks will be somewhat centered but offset to the left, half the amount that the Bookmarks button and word on the right-side takes up.

https://support.mozilla.com/en-US/kb/How+to+customize+the+toolbar
http://kb.mozillazine.org/Toolbar_customization

Did not work...still leaves the text w/ only five bookmarks showing; the rest are off the bookmark toolbar.

I found the solution from an earlier request i made in Aug., 2011...as follows:

Install the following add-on. Sounds like it will do everything you want, after you install, re-start Firefox and set the options (Add-ons > Extensions, click Options next to the add-on)

   https://addons.mozilla.org/en-US/firefox/addon/roomy-bookmarks-toolbar/ 

NOTE: the option on the Customize window for Icons (only) does not apply to the Bookmarks Toolbar.

The last time i posted this question was in early 2011...the answer i got took me to another page where i was able to minimize my bookmarks to icons ONLY & center them exactly. Your solution does not do this. When i use the customize tab & click on "use icons only", they still appear w/ text! Does someone have a better solution???

If you want to hide the text on the bookmarks toolbar and only have the icons then you can do that with code in userChrome.css

Add code to userChrome.css below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#personal-bookmarks .bookmark-item > .toolbarbutton-text { display:none!important; }