We expect to always have this accessor isNameValid. This was added in https://hg.mozilla.org/mozreview/gecko/rev/1419520f2e7ca7d41edc486e7a7f852867d253d4#index_header. Running the test devtools/client/inspector/rules/test/browser_rules_authored.js will also help check this. The inspector would not accept all names since it is checking this accessor to see if the name is valid. We are removing the guard because the accessor is available in the recent FF.
Bug 1531437 Comment 2 Edit History
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We expect to always have this accessor isNameValid. This was added in https://hg.mozilla.org/mozreview/gecko/rev/1419520f2e7ca7d41edc486e7a7f852867d253d4#index_header. Running the test devtools/client/inspector/rules/test/browser_rules_authored.js will also help check this. The inspector would not accept all names since it is checking this accessor to see if the name is valid. We are removing the guard because the accessor is available in the recent FF. You can also manually test this by throwing in some invalid declaration names when editing the rules.