Adding with javascript to admin bar. Works in Chrome/Safari, not Firefox

Obviously you cannot put a select tag inside a A anchor. What happens is related to event bubbling: Firefox takes the A anchor click into account, not the select control.

Change your html to:

 <li id="wp-admin-bar-abstractambienceantisocialapertureapzauldbackstagebig-easybiznizzbloggingstreamboastbold-newsbusy-beecaffeinatedcanvascanvas-buddypress-betachapterscinchcity-guidecodacoffee-breakcontinuumcrispcush" class="">

                            <select name="themeswitcher" onchange="location.href=this.options[this.selectedIndex].value;" style="color: #000; text-shadow: none;">
                                    <option value="http://themes.fusionized.com?wptheme=Abstract" style="color: #000; text-shadow: none;">Abstract</option>
                                    <option value="http://themes.fusionized.com?wptheme=Ambience" style="color: #000; text-shadow: none;">Ambience</option>
                            </select>

            </li>

and it should work cross-platform.