The concept: Let browser windows be synchronized with bookmark folders such that all new tabs in a window are automatically created as bookmarks in the corresponding bookmark folder.
Let all windows by default be synchronized with an unnamed bookmark folder, e.g. Unnamed/1 for the first window, Unnamed/2 for the second, etc. Now, if there should be a crash, one can restore specific pages and/or use "open in tabs" to restore whole windows.
When the current window has no web pages and the user opens a bookmark folder using "open in tabs", the window is synchronized with that existing folder.
Provide an "open in new window" feature for bookmark folders which will create a new window and open the bookmark folder synchronized with that window.
Provide a "synchronize/unsynchronize to/from folder" option under Bookmarks to (dis)associate the current window with/from a folder.
When deleting a tab which is not already explicitly saved in the synchronized folder, ask if it is to be also deleted from the folder. Provide a convenient "accelerator" for overriding this question, e.g. add SHIFT to delete from both or add CTRL to keep the bookmark while deleting the tab.
Provide a History subfolder in each bookmark folder that has ever been synchronized with a window. The History subfolder should look similar to the regular history view and record the web pages which have existed as bookmarks in that folder, which is equivalent to webpages which have been displayed in tabs of windows synchronized with that folder.
This system simplifies and generalizes session saving, gives better contextual focus for history and in general makes using the browser more focused and context-driven.