The Script class [1] is used for the following purposes: Scripts: - Static scripts (content_scripts[*].js in manifest.json, and contentScripts.register). - One-time scripts (tabs.executeScript). Style sheets: - Static style sheet (content_scripts[*].css in manifest.json, and contentScripts.register). - Add style sheet once (tabs.insertCSS). - Remove style sheet (tabs.removeCSS). The WEBEXT_CONTENT_SCRIPT_INJECTION_MS telemetry probe from is supposed to measure the execution time of content scripts. But the current implementation does not limit the measurement to scripts only. I think that we should ignore all style-only "content scripts". [1] https://searchfox.org/mozilla-central/rev/c296d5b2391c8b37374b118180b64cca66c0aa16/toolkit/components/extensions/ExtensionContent.jsm#276

> But the current implementation does not limit the measurement to scripts only. For clarity, this is the logic that measures the time of execution: https://searchfox.org/mozilla-central/rev/c296d5b2391c8b37374b118180b64cca66c0aa16/toolkit/components/extensions/ExtensionContent.jsm#486-496 Before that code block, there is logic to add/remove CSS if needed, and/or parse the script (before executing it later). The telemetry probe only measures the execution of scripts (potentially multiple, in case of static scripts...), but there is currently no guarantee that there are any scripts to execute.