Screenshot from phone, but same warning message when i do it on my computer.
I have my bookmarks, history, etc. saved on my computer (in the firefox profile folder). But i'm afraid firefox may delete my data once i reset my password. If not, will firefox sync my local data to their cloud after pw reset?
I'd recommend making a backup of your profile folder on your desktop system (close Firefox before doing so, then just copy-paste it onto an external hard-drive).
But yes, resetting your password throws away your uploaded data from Mozilla's servers, which should be identical to the data you have synced locally, which it will re-upload after the password change.
So, no data loss should occur, unless you've simultaneously lost all devices that were synced + the password.