Cache thumbnails on tab discard

[emvaized]

Many people use various tab suspend (or tab discard) addons to minimize Firefox memory usage. These addons use tabs.discard() API in order to unload tabs after set period of inactivity.

Since version 123.0 Firefox allows to show tab previews on hover when `browser.tabs.cardPreview.enabled` flag is enabled. However, thumbnails often are not shown for tabs which were previously discarded by 3rd party addons.

Would be great if Firefox implemented some mechanism under the hood, which will automatically cache last "active" view of the tab whenever it is requested to be suspended by `tabs.discard()` API, and then show the cached thumbnail on hover. Would also be great if cached version will be returned to 3rd party addons on tabs.captureTab() API requests as well.

[Jon]

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