A Saved Search folder looks like a regular mail folder/label, but when you click on it, it runs a search according to criteria that you’ve set previously and it displays a list of messages that match those criteria. A Saved Search is a “virtual folder” in the sense that it merely displays a set of messages that meet the search criteria, while the actual messages remain stored elsewhere. If you select and delete a message inside a Saved Search it will get deleted from its actual location, but if you delete a Saved Search folder itself all of the actual messages will remain intact. Moreover if you modify the search criteria for a Saved Search its virtual “contents” will be accordingly updated. Thunderbird has it. Now Gmail has it too. (Adding Persistent Searches to Gmail)
The only disadvantage of it is that you cannot take it with you as is related to the browser and is not a plug included in the Gmail features. But as the guy that did it works for Google, they might notice the script and plug it in the Gmail standard features.