Damon,
While searching for posts discussing zoomin, I came across this unanswered one.
A quick test shows the same experience you had, except for me on Firefox 66.0.2 the lower right corner does have the resizer, and clicking it resizes the edit area back to normal.
I don't know if anyone will pick up on this issue, and it is a little beyond my capability to delve into!
Regards