Let me throw out some additional ideas. We could potentially use a page-icon: uri when we restore a tab without a defined icon. Then, if the profile is not in permanent PB mode and history is enabled, we could avoid storing the favicon. That would fetch the favicon from Places on restore. Advantages: we save a lot of bytes. Disadvantages: the risk of not having an icon increases a bit, porting the session to a different profile may miss a lot of icons (This is probably the deal breaker). We could store the favicon screen representation instead of the downloaded favicon data, potentially in webp format, if it's saving space. Advantages: we store smaller optimized images. Disadvantage: we must encode an image per each tab.
Bug 1849393 Comment 30 Edit History
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Let me throw out some additional ideas. We could potentially use a page-icon: uri when we restore a tab without a defined icon. Then, if the profile is not in permanent PB mode and history is enabled, we could avoid storing the favicon. That would fetch the favicon from Places on restore. Advantages: we save a lot of bytes. Disadvantages: the risk of not having an icon increases a bit, porting the session to a different profile may miss a lot of icons (This is probably the deal breaker), icon may miss indicators (like count of unread mails). We could store the favicon screen representation instead of the downloaded favicon data, potentially in webp format, if it's saving space. Advantages: we store smaller optimized images. Disadvantage: we must encode an image per each tab.