Search in Popup is a Firefox extension that opens a popup of a search page using the currently selected text. It is similar but more limited than the Selection Search addon, but overcomes that addon’s limitation of not having a popup functionality.
Source code: It is open sourced on Github.
Install: Go to its Firefox addons page.
Usage: Select text on a webpage, right-click and from the context menu choose Search in Popup and one of the listed search engines. The search webpage opens in a popup at the top-right corner from the icon of the addon.
Search engines: In the settings, add a search
engine by providing a name, its domain name, its query string and always
select the extended query option. In the query URL,
{q} is the placeholder for the selected text. For example,
to search for a book in my local library, I provide the URL as
https://sjpl.bibliocommons.com and the query string as
/v2/search?query={q}.