How to revert back to old update without losing data?

Using your backup from that time, or restore point to roll back to. Downgrading your current profile might cause data loss (e.g. password storage changed around v155) and introduce integrity issues that could exhibit later on. The manual steps are basically covered with [[Restore bookmarks, passwords and data from an old Firefox profile]] where you migrate the contents of the various data types piecemeal and check after each step these work as expected, before moving on to the next one. This way you'll know where you hit any compatibility limits. There's no longer any supported version older than v153 so your mileage may vary.