All modern web browsers leak extension information to sites if the sites run scripts to pull the information. We talked about the findings of a research term that published its findings recently in a paper.
Unless scripts are blocked, sites may run scripts that check the response time of the browser as it is different when checks are made for fake extensions and fake resources, and existing extensions and fake resources.
Firefox's situation is special, as it supports the
Anyways, the Firefox screenshot tool is inadequate — it's better to disable it completely (as follows) and use a full-fledged, standalone screenshot tool, instead:—
("extensions.screenshots.disabled", true);
("extensions.screenshots.system-disabled", true);