I have a Mikrotik router device.
Which webpage shows the changelog of a new/old firmware update?

A search didn't yield results.

MikroTik removed firmware changelog (not ROS changelog) years ago, no idea why.

Please look there: https://mikrotik.com/download/changelogs

If you're talking about routerboot changes ... then the changelog, linked by @memelchenkov in post above, contains a few changes in routerboot category. However, I've got a feeling that the lust is not really complete ...