Unfortunately, Firefox has no built in way to rearrange your toolbars. It's on the plans for a future release, but in the mean time you can do it manually. This trick will only help you arrange toolbars loaded by plugins, such as the Google Toolbar and the Web Developer toolbar. The default File, navigation, and bookmarks toolbars will not be rearranged.

Navigate to your Firefox profile folder. It will be located in C://Documents and Settings/[username]/Application Data/MozillaFirefox/Profiles/[something].default

In this folder you should find extensions.ini. Open it.

You will see some lines that look like:

Firefox loads the toolbars for your extensions in reverse order. Extension2 will be the top toolbar and Extension0 will be the bottom toolbar. Simple rearrange and renumber these lines. When you restart Firefox, the toolbars will have changed order.

Note: You can see what the name of the extention is by looking in the extensions {3112ca9c-de6d-4884-a869-9855de68056c} chrome folder.