When a rich text field's Web Access: Display property is marked "Best Fit For OS", the web server selects one of the HTML editors based on the browser.
The Java Editor Applet is used for all browsers but Internet Explorer (IE).
As of 8.5.1 FP1 the domino-data-directory
/browser.cnf file can be used to control the Best Fit For OS.
Look at the file for the "RTEditorBestFit" property. There is a comment in the file explaining its use.
Put the following in browser.cnf
Property RTEditorBestFit String Legacy
Rule JSControl MSIE [6-9].* Windows NT [6-9]
Rule JSControl Firefox/[3-9]
Rule JSControl Version/[3-9][0-9\.]* Safari
Rule JSControl Chrome/[2-9]
Rule JSControl Opera/[7-9]
Note: you should make a backup copy of the browser.cnf file, in case it is overwritten when you upgrade to a new domino server version.
See also Extending Dojo Rich Text Editor's Toolbar