Hide Google Search Experiment Survey - Firefox

Google Search Survey Box Removal in Firefox
Update: It appears Google has since removed the survey box. This change occurred relatively quickly, demonstrating Google’s responsiveness.
Users participating in Google’s Experimental Search programs, such as the keyboard shortcuts test, may have encountered a persistent survey box. This box often reappears even after the survey has been completed. Fortunately, the Firefox browser’s customization options allow for easy removal of this element using the Stylish extension.
Using the Stylish Extension
The Stylish extension is required for this modification to function. It is a highly recommended extension for browser customization. To begin, click the Stylish icon located in the browser’s status bar.
Select "Write Style" from the dropdown menu, then choose "For google.com..." while viewing the Google search results page. Alternatively, select "Blank Style" if you are not currently on the search results page.

Paste the following code into the provided textbox. A descriptive name for the style should also be entered in the description box.
@namespace url(http://www.w3.org/1999/xhtml); @-moz-document domain("google.com") {
#exp_msgs {display:none !important;}
}

Clicking either "Save" or "Preview" will conceal the unwanted survey box.

Further Customization with Stylish
The Stylish extension offers extensive customization capabilities. It can be utilized to hide any undesirable element on any webpage. Furthermore, it allows for complete visual overhauls of entire websites.
For a vast collection of user-submitted styles, consider exploring the resources available at userstyles.org.