LanguageTool Slow and Using High Memory in Firefox

How to fix it

// try these in order, the most common fix is first

1. FIX 1Roll back to version 9.0.1

   Copy steps
   1. 01Open the LanguageTool page on addons.mozilla.org
   2. 02Scroll to 'See all versions' and install 9.0.1
   3. 03Right-click the LanguageTool icon > Manage Extension
   4. 04Set 'Allow automatic updates' to Off
   Did this fix it? Yes No

2. FIX 2Disable the extension on heavy sites

   Copy steps
   1. 01Click the LanguageTool icon on the slow page
   2. 02Turn checking off for this site
   3. 03Reload the tab
   Did this fix it? Yes No

3. FIX 3Close the triggering tab

   Copy steps
   1. 01Open about:processes to see which tab uses memory
   2. 02Close it or restart Firefox to release RAM
   Did this fix it? Yes No

4. FIX 4Update when a fix ships

   1. 01Check about:addons for new versions; version 11 shipped in mid-2026 without a changelog, so test it before relying on it
   Did this fix it? Yes No

Symptoms

Firefox freezes or crashes while typing, the Extensions process uses huge CPU or tens of GB of RAM, typed letters appear with a long delay.

Why it happens

• C-01Bug in LanguageTool extension 10.0.9 and later (GitHub issue still open)
• C-02Very large text fields or rich editors (e.g. CKEditor) being checked continuously

// this guide also covers

• languagetool firefox memory
• languagetool slow firefox
• languagetool extension crashing browser
• languagetool high cpu

The same problem in other apps

All slow, laggy or resource-hungry guides →

• Grammarly Slowing Down Your Browser or Computer (Grammarly)
• Why Is QuillBot So Slow (QuillBot)
• Why Is ProWritingAid So Slow? (ProWritingAid)
• Hemingway Editor Slow With Long Documents (Hemingway Editor)

Other LanguageTool errors

• LanguageTool Extension Stopped Working (Upgrade Required)
• LanguageTool Not Working in Google Docs
• LanguageTool Not Working in Word
• LanguageTool Detects the Wrong Language
• LanguageTool Not Working in Overleaf
• LanguageTool Not Working in Chrome
• LanguageTool Text Too Long (Character Limit)
• LanguageTool Self-Hosted Server Setup (Use the Extension Free)