Firefox's session is stored entirely within sessionstore.jsonlz4 in the profile folder (and backups, or while running, in sessionstore-backups) -- locate using about:profiles. It's a nonstandard lz4 container but you can find implementations, e.g. lz4json/lz4jsoncat in some package managers.

Once decompressed you have json lists of windows of tabs, including recently closed; for every tab, urls and titles of each entry in per-tab history, base64ed favicon, last selected time, and various less useful data (some still incomprehensible -- binary serialized or cache keys?). Cookies and data from extensions like treestyletabs are in there too.

OTOH... is there a currently working deserializer for Chrome's session files?