If you have vista, you should definitely try out glasser, which gives you aero effects in the nav-bar. I've redone this theme for glasser, and will be doing future updates there:
Len R's rounded FF3 for glasser

A lot of work went into this - if you appreciate it, please support my work.
v1.2

Only tested on Windows. Please post screenshots and post in forums for bugs.
====>IMPORTANT: Unfortunately, it is impossible to code a CSS theme that works right on both windows and linux. So, this is a WINDOWS theme. For a linux theme, please see vbielawski's work.

Show rounded text fields, menulists and more in Firefox 3
Includes webpage fields and multiline fields
shows blue hover and focus for all fields (vista-themed colors)

I've integrated parts my vista statusbar and findbar theming upon (many) requests;
also shows solo images (if you click "view image") centered, shows wider google results (full-width) - all easy to comment out if you don't like them.

Based on concepts from vbielawski's ex