The complete set of action performed by this add-on on installation are:
- reset "browser.startup.homepage" pref to its default value
- reset "keyword.URL" pref to its default value
- reset "browser.search.defaultenginename" pref to its default value
- new in version 0.2: resets browser.newtab.url to its default value
- restores the build's default search engine if it was removed
- moves the build's default search engine into first position and selects it
Note that it's possible for a given pref's "default value" to be modified by an add-on. To be sure that you're getting Firefox's original defaults, you should also carefully audit your installed add-ons.
The code is available at https://github.com/gavinsharp/SearchReset . There is no easy way to undo the changes, but the old pref values are saved and can be viewed using about:config (search for "searchreset.backup.").