Did you check the Graphics section on the about:support page for WebGL related error messages ?

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

You can also check the Browser Console.

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/docs/Tools/Browser_Console

No problem with Graphics. My only problem : How to lock "webgl.min_capability_mode" on false ?

See:

• https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

You can check for problems with preferences. You can delete possible user.js and numbered prefs-##.js files and rename/remove the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thanks ! Finally, I used about:profiles Open "répertoire racine" Open user.js Delete the line webgl.min_capability_mode Save the file So, I can change webgl : false. And my preference is saved ! I can read webgl ! It's ok !

What else is in this user.js file ?

If you didn't create this user.js file yourself then this file wouldn't be present and you can remove this file.

In user.js : 162 lines ! I didn't create it.... I used it to activate "reader mode"..... And perhaps for other pref in the future ?

I think that best is to remove this file if it contains that many lines. The user.js is processed each time you start Firefox, so you can only change prefs set via user.js for the current session and you lose changes when you close Firefox.