Thanks for reporting! This relates to Map Scopes feature not been switched on. See the screen shot. Once the box is checked the preview value should show correctly. Also see the documentation https://firefox-source-docs.mozilla.org/devtools-user/debugger/using_the_debugger_map_scopes_feature/index.html
Bug 1680228 Comment 3 Edit History
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Thanks for reporting! This relates to Map Scopes feature not been switched on. See the screen shot. Once the box is checked the preview value should show correctly. Also see the documentation https://firefox-source-docs.mozilla.org/devtools-user/debugger/using_the_debugger_map_scopes_feature/index.html It's clear this feature might not be too discoverable by users, we'll look into improving this See https://bugzilla.mozilla.org/show_bug.cgi?id=1695621#c1 for details.