Firefox Privacy Extensions: Which Data Leaves Your Browser and Which Stays Local

Not all extensions handle your data the same way. Some send data to external servers; some process everything locally. Here is a clear breakdown for extensions that handle your content, writing, or browsing context.

Extensions that process your content locally

Grammar and writing:

AI Writing Assistant for Firefox — Grammar checking and text rewriting runs in the browser using JavaScript. No text is sent to any server. For sensitive documents, this is the meaningful choice over cloud grammar tools.

Voice recording:

Voice Notes and Audio Recorder — Uses Firefox's Web Audio API. Audio is captured locally and saved as a file on your machine. Nothing is uploaded.

Task management:

Daily Planner, Habit Tracker — Tasks and habit logs stored in Firefox's local extension storage. Nothing is synced or transmitted.

Extensions that make external network requests

Weather:

Weather and Clock Dashboard — Makes a weather API request to fetch current conditions. Sends your location (city name or coordinates) to a weather service. The request is minimal: location in, weather data out. No tracking.

Stock data:

Stock Market Dashboard — Fetches stock prices from a public market data API. Sends your symbol list to fetch prices. No account linking, no personal data.

News headlines:

News Widget Dashboard — Fetches RSS feeds from URLs you configure. The only data transmitted is the RSS feed URL you chose — which is public by design.

Extensions with no external connections

Display and navigation:

NightOwl, Speed Dial Hub, Tab Manager, Quick Calculator — These extensions make no external network requests. All functionality is local.

Summary table

All 11 are part of a series of Firefox extensions I built — see the full series.