The https://collector.openwebdocs.org/ project (v10.17.11) found new features shipping in Firefox 151 beta which was released yesterday. Currently, the collector covers about 90% of BCD, so the following list might not be exhaustive. Also, if a feature is in Firefox Nightly only, it is not considered here.

With this PR, BCD considers the following features as shipping in Firefox 151 (bold features are new keys in BCD).

• api.CSSContainerRule.conditions (ref)
• api.CanvasRenderingContext2D.lang (ref)
• api.DocumentPictureInPicture (ref)
• api.DocumentPictureInPicture.enter_event (ref)
• api.DocumentPictureInPicture.requestWindow (ref)
• api.DocumentPictureInPicture.requestWindow.option_disallowReturnToOpener (ref)
• api.DocumentPictureInPicture.requestWindow.option_preferInitialWindowPlacement (ref)
• api.DocumentPictureInPicture.window (ref)
• api.DocumentPictureInPictureEvent (ref)
• api.DocumentPictureInPictureEvent.DocumentPictureInPictureEvent (ref)
• api.DocumentPictureInPictureEvent.window (ref)
• api.GPURenderBundleEncoder.setVertexBuffer.unset_vertex_buffer
• api.GPURenderPassEncoder.setVertexBuffer.unset_vertex_buffer
• api.GPUSupportedLimits.maxStorageBuffersInFragmentStage (ref)
• api.GPUSupportedLimits.maxStorageBuffersInVertexStage (ref)
• api.GPUSupportedLimits.maxStorageTexturesInFragmentStage (ref)
• api.GPUSupportedLimits.maxStorageTexturesInVertexStage (ref)
• api.HTMLTemplateElement.shadowRootSlotAssignment (ref)
• api.Navigator.serial (ref)
• api.OffscreenCanvasRenderingContext2D.lang (ref)
• api.Serial (ref)
• api.Serial.getPorts (ref)
• api.Serial.requestPort (ref)
• api.SerialPort (ref)
• api.SerialPort.close (ref)
• api.SerialPort.connect_event (ref)
• api.SerialPort.connected (ref)
• api.SerialPort.disconnect_event (ref)
• api.SerialPort.forget (ref)
• api.SerialPort.getInfo (ref)
• api.SerialPort.getSignals (ref)
• api.SerialPort.open (ref)
• api.SerialPort.readable (ref)
• api.SerialPort.setSignals (ref)
• api.SerialPort.writable (ref)
• api.Window.documentPictureInPicture (ref)
• api.WorkerNavigator.serial (ref)
• css.properties.position-anchor.normal (ref)
• html.elements.iframe.allow.serial (ref)

Updated in a different PR:

• webdriver.bidi.browser.setClientWindowState
• webdriver.bidi.browser.setClientWindowState.clientWindow_parameter
• webdriver.bidi.browser.setClientWindowState.height_parameter
• webdriver.bidi.browser.setClientWindowState.state_parameter
• webdriver.bidi.browser.setClientWindowState.width_parameter
• webdriver.bidi.browser.setClientWindowState.x_parameter
• webdriver.bidi.browser.setClientWindowState.y_parameter
• webdriver.bidi.script.getRealms.type_parameter.worker