[Bug] Cannot position cursor and cannot select function in editor (FireFox)

[MarcoDieleman]

In FireFox we cannot position our cursor at the beginning of the page or between functions in the HTML-editor (if the content already contains functions). We cannot select a function and delete it. We can still double click a function to edit it.

Tested with C1 version 6.8 en 6.10, FireFox 88.0