Ignore the version error. Per
Sideloading and debugging/WebIDE on BananaHackers Wiki, "[...], you may now see a warning header about mismatched build date. You can safely ignore it as WebIDE was mainly designed to support Firefox OS device builds released alongside that Firefox/Pale Moon versions." You've got the phone connected, and it'll show this every time. Now, click on Device Settings by the right pane, search for
callrecording.mode entry and switch it to either
on (press D-Pad Left to record each time you make a call) or
auto.
As I've said above, WebIDE in Firefox Nightly and more recent variants of Waterfox is designed for newer B2G versions found in KaiOS 3, and not KaiOS 2.5.3.