I thought I should let you know that the issue with Fire Fox and the Editor, where it is unable to get focus if not shown on the page initially is still present in Firefox 68.4.1esr. This is an asp.net core 3.1 app using UI for asp.net Core v2019.3.1023
The js refresh fix as described here https://www.telerik.com/forums/editor-issues-with-firefox-17 fix still works.
For me, I have the Editor embedded in a Tab page and I am triggering the refresh within the onShow client event.
I'm not sure if there is anything you can do to address this as it seems to be a Firefox issue. I did find a bugzilla reference mentioned in a post somewhere but that points to a very old closed bug: https://bugzilla.mozilla.org/show_bug.cgi?id=254144