In MV3, the browser_action API has been renamed to the action API (bug 1706398). In Chrome, the action API supersedes even the page_action API (under consideration in bug 1687756).

As part of the rename, the "_execute_browser_action" keyword has been renamed to "_execute_action". Firefox supports this in the commands API *. Users can customize the shortcut registration via UI in about:addons, which is persisted, keyed by the shortcut ID. If extension authors are forced to migrate to _execute_action, then the key changes, and users would lose their customized shortcut.

To avoid the loss of user customizations, we should consider migrating the _execute_browser_action shortcut to _execute_action (or somehow use it as a fallback), at https://searchfox.org/mozilla-central/rev/2d1097c27f31cb15f7588b043d691cda9710d14b/toolkit/components/extensions/ExtensionShortcuts.jsm#292-301.
To avoid the fallback from unexpectedly popping up, we should clear both _execute_browser_action and _execute_action when the shortcut is reset (resetCommand implementation).

* command is also supported in the menus / contextMenus API. I'm not concerned about the menu registrations, because these do not persist beyond extension updates nor browser updates.

https://hg.mozilla.org/mozilla-central/rev/adf333b4e360

Should be added to the changelog of Firefox 127 for developers.

Content changes are ready for review

• MDN Fallback for _execute_browser_action #34091
• EW MV3 guide note for fallback for _execute_browser_action #1886