Saved search folder (Like Thunderbird) for multiple accounts
Basically, filters for emails in multiple accounts that can be show in the sidebar. This feature is similar to a unified inbox and search folder, but for common searches you do. For example, I could open a search folder that would search for emails with the word "support ticket #" in the subject field from multiple accounts.
Please refer to https://support.mozilla.org/en-us/kb/using-saved-searches .