Please note that this extension is superceded by a new one based on the WebExtensions API for Firefox 57 and later: https://addons.mozilla.org/en-US/firefox/addon/recoll-we/
The source for the new version is here:
https://opensourceprojects.eu/p/recollwe/code/
This Firefox extension allows you to include the web pages that you visit in Firefox in the index built by the Recoll text search tool.
The extension has been reviewed and should now be directly installed from the Mozilla addons...