I'm not fully sure if adding early escape in _resizeEditor method is the best approach. This method is called in two places:

• setContent()
• postRender() -> attached as event listener to input event (https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/e8f5de362dfb59a0cf179217d8b2e455ef15dba9/src/mobile.editor.overlay/EditorOverlay.js#L237).

IMHO it would be better to prevent the latter only, we shouldn't resize on each input event (which causes the editor to jump). Resizing the overlay on setContent() might be required, otherwise, the editor might not match the device size. Please verify that.

I've been poking around this issue today, and it seems like exiting early in _resizeEditor leaves the editor in a wonky looking state (half the screen blank where the keyboard would be)
but that does fix the issue.
Removing the input event-binding in postRender however, does not fix the issue, which seems strange to me, because it suggests that event is still firing, but I'm not sure where (on setContent?, I don't see the event in the logs, so I'm not sure if the root cause is really _resizeEditor).

Change 494240 had a related patch set uploaded (by Jdrewniak; owner: Jdrewniak):
[mediawiki/extensions/MobileFrontend@master] Disabling editor resizing on Firefox

https://gerrit.wikimedia.org/r/494240

Change 494240 merged by jenkins-bot:
[mediawiki/extensions/MobileFrontend@master] Disabling editor resizing on Firefox

https://gerrit.wikimedia.org/r/494240

@Jdrewniak I pushed a slightly modified version of your patch with Stephen today to make sure the bug fix went out in this week's train. Feel free to improve on that patch at your leisure or call this done. T214880 will remain open :)