You can set the width and the maximum widths of the Firefox search bar (the Google/Wiki bar) by performing the following steps:

1. Locate userChrome-Example.css and rename to userChrome.css in your profile directory (under Windows, it’s usually in:
   C:\Documents and Settings\[user]\Application Data\Mozilla\Firefox\profiles\[profile dir]\chrome\
   (Note: some directories might be hidden, so you’ll have to run explorer with the “show hidden files” option)
2. Insert the following in that file:
   #search-container {
width: 400px !important;
max-width: 400px !important;
}
3. Save the file, and restart Firefox. Voila, bigger search bar!