Firefox Extensions That Work Offline: Grammar, Voice, Habits, and Math Without a Backend
Most productivity tools are cloud-first. Notes sync to servers. Grammar checks go through APIs. Habit trackers call home. This is convenient when it works and a privacy concern when you think about it.
A category of tools does not need a backend at all: anything that processes or stores personal data that never needs to leave your machine.
Firefox extensions that work entirely offline
Here are four extensions I built that have no backend, no account requirement, and no network calls for core functionality.
1. AI Writing Assistant
Grammar checking and text rewriting, processed entirely in Firefox. No text is sent to an external server.
This is the meaningful privacy benefit: other grammar tools (Grammarly, LanguageTool cloud) intercept your typing and send it to their servers. AI Writing Assistant processes locally.
AI Writing Assistant for Firefox
2. Voice Notes and Audio Recorder
Records your microphone and saves the audio locally as an MP3. The file never touches a server.
Compare this to web-based voice recording apps where your audio goes to their infrastructure. Voice Notes captures via Firefox's Web Audio API and saves the file to your Downloads folder.
Voice Notes and Audio Recorder for Firefox
3. Habit Tracker New Tab
Stores your habit data in Firefox's extension storage. No sync, no account, no server. The grid is on your machine only.
This is the right choice for personal data like health habits, medication tracking, or behavioral patterns that you would not want in a cloud database.
Habit Tracker New Tab for Firefox
4. Quick Calculator and Unit Converter
Basic math and unit conversion in the browser, fully offline. Currency conversion makes one API call to get exchange rates, but the calculator itself works without network access.
Quick Calculator and Unit Converter for Firefox
Why local matters for personal tools
The convenience of cloud sync is real. The privacy risk is also real. For tools that process personal data — your writing, your voice, your health habits — a local-first approach means:
- The data exists only on your device
- No company can be breached to expose your data
- The tool works without internet access
- No account to delete if you change tools
For productivity tools where the data is personal and the network syncing is not strictly necessary, local-first is the right default.
These are four of 11 Firefox extensions I built — all free, no-account, no-server. See the full series for the others.