SMS & MMS (plus call logs) can be backed up and restored with
SMS Backup & Restore or
My Backup.
Chrome bookmarks should be synced to your Google account. Go to Settings->Accounts->Google, then select your primary Google account and enable "Sync Chrome." Then, open Chrome, then tap the three vertical dots in the upper right to open the menu. Scroll to Settings, tap your name at the top (this should match your primary Google account). Tap "Sync" and then select the options you want to back up to your Google account.
Firefox has its own syncing capabilities, too, but you'll have to create an account with them in order to manage it.
Do the following for WhatsApp: open the app, then open the three-dots menu in the upper right and select "Settings." Tap "Chats," then "Chat backup." That's where you'll find the option to back up your chats to Google Drive.
How are you planning on getting the phone to roll back to Android 6? Resetting to factory defaults doesn't revert to pre-update versions of Android - it just erases internal (and if you choose, external) storage devices.