Most grammar tools process your text on remote servers. Grammarly, LanguageTool (default configuration), and similar tools send your text to their APIs for analysis. For sensitive content, this creates a privacy consideration.
Here is how to check grammar in Firefox without cloud processing.
AI Writing Assistant — Firefox Add-ons
How it works
The extension runs a grammar model locally inside Firefox. When you request a check, your text is processed by JavaScript running in the browser — no network request is made to any external grammar service.
Setup
- Install from Firefox Add-ons
- The extension activates automatically
- On any page, highlight text, right-click, and select the writing assistant
No configuration, no account.
What it checks
- Subject-verb agreement
- Sentence structure
- Common grammar errors
- Passive voice suggestions
- Rewriting suggestions for clarity
Accuracy trade-off
Cloud grammar tools (Grammarly, LanguageTool) run larger models with more training data. Local models running in a browser are smaller and may miss some edge cases.
For standard grammar checking — business emails, blog posts, documentation — local accuracy is sufficient. For polished professional writing where you need every edge case caught, a cloud tool is more capable.
When local matters
- Legal drafts: Attorney-client privileged content
- Medical notes: Patient information
- Financial documents: Non-public information
- HR communications: Confidential employment matters
- Personal writing: Journal entries, private correspondence
For any of these, sending the text through a third-party API is a risk. Local processing eliminates it.
Install AI Writing Assistant for Firefox
AI Writing Assistant is one of 11 Firefox extensions I built — see the full series.
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