I amended the file "nsSearchService.js" to introduce two new functions to the Google search bar in FireFox:
1- Search the current site with the syntax "site:this"

2- Conduct a Google Image Search with the syntax "img:"

To install this hack, re-name your current 'nsSearchService.js' file (just in case you want to remove the hack) and replace the original with this one:
New and improved 'nsSearchService.js'. See lines 733 and 855 for the edits.
On Vista (and presumably XP), 'nsSearchService.js' is located in this directory: C:\Program Files\Mozilla Firefox\components
On Linux, the file is located here: /usr/lib/firefox/components/nsSearchService.js
This solution is not very modular; the file must be installed or deleted manually. However, it does offer some advantages:
- Functionality is directly integrated into the Google search bar so there is no need for a
separate search engine- No need for a
custom bookmark/search via the location bar --- this keeps the suggestions from the Google tool bar in tact. (You can add the special syntax to the end of your search terms.)
- No need for a
special button; just type the command in the search bar.
- No need for
Google's Advanced Toolbar for Firefox. (Google's Advanced toolbar has buttons for searching the current site and conducting an image search; however, there is no syntax that I know of to automate this procedure. In addition, for searching the current site, the regular expression used in my hack removes the 'www' from the current domain which enables a broader search. Google's Advanced toolbar does not do this; it searches 'www.stanford.edu' rather than 'stanford.edu'.)