Grammar checking is one of the most-used browser extension categories. Grammarly, LanguageTool, Hemingway — they all exist because people write in browsers all day and want editing assistance without switching apps.
The common thread across the popular options: they send your text to a server for processing.
What "sends text to a server" actually means
When Grammarly processes your text, the browser extension captures what you type and sends it to Grammarly's servers. This is how the cloud-based AI model works — the model runs on their infrastructure, not your device.
Grammarly's privacy policy describes this. LanguageTool's cloud extension does the same. This is not a hidden behavior — it is how cloud-based tools work.
For most writing (emails, blog posts, general documents), this is fine. For some writing, it is not:
- Legal documents referencing client details
- Medical notes with patient information
- Internal business documents marked confidential
- Personal correspondence containing sensitive information
- Journalism about sources or unreleased information
AI Writing Assistant for Firefox
AI Writing Assistant processes text locally, inside Firefox. The grammar checking and rewriting runs in JavaScript in the browser. Text does not leave the device.
Features:
- Grammar and spelling suggestions
- Sentence rewriting (clarity and concision)
- Tone adjustment (formal/informal)
- Text expansion (brief note → full paragraph)
- Simplification (complex → plain language)
How it works: A local language model (compressed for browser use) processes the text in the Firefox extension context. No network call is made for text processing.
AI Writing Assistant for Firefox — Firefox Add-ons
The honest trade-off
Grammarly's cloud model is more powerful than what can run locally in a browser. Suggestions from cloud-based tools are likely more accurate and nuanced.
For users where accuracy is the priority and privacy is not a concern, Grammarly remains the better choice.
For users editing sensitive content, accuracy takes a back seat to containment. AI Writing Assistant is the right tool when the content cannot leave the browser.
AI Writing Assistant is one of 11 Firefox extensions I built — see the full series.
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