Grammarly is the market-leading grammar tool. It has 30 million daily active users. If you have used grammar checking in the browser, you have probably used Grammarly.
The core product requires installing a browser extension that reads your keystrokes and sends text to Grammarly's servers for analysis. This is how it works — cloud processing enables their AI model to run. It is also documented in their privacy policy.
For users who want grammar checking without cloud processing, here are the alternatives.
LanguageTool (browser extension)
How it works: LanguageTool offers a self-hosted option, but the browser extension uses their cloud API by default. Like Grammarly, text is processed server-side.
Privacy: cloud-based by default; self-hosted available but requires running your own server
Accuracy: Generally considered comparable to or slightly below Grammarly
Cost: Free tier with limited suggestions; premium from $4.99/month
Hemingway Editor (web app)
How it works: Web-based editor. Paste in text, it highlights issues. Entirely in-browser — no account required, no server processing of your text.
Privacy: processes locally in the browser
Accuracy: Style-focused (readability, passive voice, complex sentences); not a grammar checker in the traditional sense
Cost: Free for web; $19 one-time for desktop app
AI Writing Assistant for Firefox (browser extension)
How it works: Firefox extension that provides grammar checking and text rewriting inside the browser. Processing happens locally — no text is sent to external servers.
Privacy: local processing only; nothing leaves the browser
Accuracy: locally-processed, so the model is smaller than Grammarly's cloud model; sufficient for most use cases
Cost: Free
AI Writing Assistant for Firefox
Comparison
| Tool | Processing | Account | Cost |
|---|---|---|---|
| Grammarly | Cloud | Required | Free / $12/month |
| LanguageTool | Cloud (default) | Optional | Free / $5/month |
| Hemingway | Local (web app) | No | Free |
| AI Writing Assistant | Local (extension) | No | Free |
When local processing matters
If you write about sensitive topics — medical, legal, financial, or personal content — sending that text to a third-party server for grammar checking creates exposure. Even services with strong privacy policies are processing your text on infrastructure they control.
For users in this situation, a local grammar tool is the right choice. The accuracy trade-off (smaller local model vs. larger cloud model) is worth it when the alternative is exposing sensitive content.
For general writing where privacy is less of a concern, Grammarly remains the most accurate tool.
AI Writing Assistant is one of 11 Firefox extensions I built — see the full series.
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