Firefox: Add a custom Google search including pre-defined keywords

I'd like to figure out a way to have customized Google searches to choose from in the Firefox search bar in the top right corner. By customized I just mean the possibility to add specific pre-defined keywords which would then be appended automatically to the performed Google search.

For example, if I am looking for movie subtitles, I'd just like to be able to select my custom made "Google Subtitles Search" that would just magically add a "subtitle" keyword to the search.

The Mycroft Project generator should help you generate plugins that will add custom search engines to your browser. The linked page is well-documented itself.

There is a way to use the address bar to search various websites (works for every input field!), which is much faster to use than using the custom search bar.

It saves the searches as bookmarks, which allows you to edit the bookmark and change its URL to insert anything you want.

Example

A search bookmark for DuckDuckGo might have this URL (where %s is the placeholder for your search):

https://duckduckgo.com/?q=%s

Now you could copy this bookmark entry, set a different search keyword and change the URL so that it only searches pages on superuser.com (by using the site: operator):

https://duckduckgo.com/?q=site:superuser.com %s