if anything, turning off the phone during those 8 hours would be worse, as starting back up takes a surprisingly high amount of battery. but inbetween shifts it'll likely help to turn off the phone as long as you are sure you won't need the phone at all during that time
as one of the other comments mentioned, enable airplane mode to save a good bit of battery. while there is an app meant to help in simmilar scenarios, ForceDoze, it's annoying to set up and requires some quite nerdy stuff.
to save your tabs in case of a crash or for the "restore tabs on open" option
you can disable that or reduce how frequent it is in firefox about:config, but on chromium too bad