The dictionary preferences are stored per-site and the sanitize-on-shutdown mechanism clears them as part of the "site settings" category: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/browser/modules/Sanitizer.jsm#529,536 The content pref service is cleared by the ClearDataService here: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/toolkit/components/cleardata/ClearDataService.jsm#1104,1122 This seems like intended behavior, since you explictly request Firefox to clear "site settings" which clears site-specific preferences. If you don't want the configured dictionaries to be cleared I suggest to uncheck this option in preferences. This is a "works as intended" from the data sanitization perspective, but I'll let the folks working on dictionaries consider whether we selected dictionary pref should be a global setting (if it isn't already somewhere?). Searchfox points to DOM: Editor for this file: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/editor/spellchecker/EditorSpellCheck.cpp#75. Please move the bug if that's not the correct component.
Bug 1777616 Comment 4 Edit History
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The dictionary preferences are stored per-site and the sanitize-on-shutdown mechanism clears them as part of the "site settings" category: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/browser/modules/Sanitizer.jsm#529,536 The content pref service is cleared by the ClearDataService here: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/toolkit/components/cleardata/ClearDataService.jsm#1104,1122 This seems like intended behavior, since you explictly request Firefox to clear "site settings" which clears site-specific preferences. If you don't want the configured dictionaries to be cleared I suggest to uncheck this option in preferences. This is a "works as intended" from the data sanitization perspective, but I'll let the folks working on dictionaries consider whether the selected dictionary pref should be a global setting (if it isn't already somewhere?). Searchfox points to DOM: Editor for this file: https://searchfox.org/mozilla-central/rev/fa71140041c5401b80a11f099cc0cd0653295e2c/editor/spellchecker/EditorSpellCheck.cpp#75. Please move the bug if that's not the correct component.