VimFx is a Firefox extension that allow you to click on a link or on a text field with f, and to enter visual mode on plain text with v.
To jump to a CodeMirror editor one would expect to use f rather than v, because a CodeMirror editor is an editable text zone.
f is indeed working perfectly to edit CodeMirror's demo.
However, I cannot use f on Compiler-Explorer UI (live demo).
The problem can probably be fixed by changing the HTML code that embed CodeMirror.
Note that v is working in this case, but does not satisfy me as I would like to enable CodeMirror's vim key bindings.
This has been fixed in VimFx as of commit 62f2f030, and is available in version 0.19.0.