Firefox 1.0PR broke the old way of making the search box wider. The new way is to add this to your userChrome.css file (I like my search box BIG, you can adjust the number to suit):
/* Make the Search box flex wider */
#search-container {
-moz-box-flex: 600 !important;
}
#searchbar {
-moz-box-flex: 600 !important;
}
This is much easier if you get the chromEdit extension, which allows you to edit userChrome.css, userContent.css or user.js, without the hassle of finding your profile.