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?
Indeed! Dumping this recovery.jsonlz4 file on the command line is how I inspect and archive all my tabs and windows. You can do this on remote systems, too, and Firefox does not need to be running.
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?