Those file behavior file changes are under the control of the OS. That's where you need to look.

No, because v127 and before open an dialog window and offer to choose the application to open for unknown files, after v127 not. Firefox ignore all this after v127 since local file are not hosted by server and have no HTTP Header like Content-Type.

I wouldn’t ask if the behavior hadn’t changed after version 127. I’m asking if it’s something native built-in or can it be changed by some setting?

Actually as workaround it would be also enough for me if i can put something to url like.

from

file:///home/..../index.rst

to

open-default-appication:file:///home/..../index.rst

to achieve the desired behavior...

Like it's done for view-source:file:///home/..../index.rst (in case if i wish the source code)

Ok I found acceptable solution for me

   * about:config 
   * set view_source.editor.external to true by double-clicking on it.
   * set view_source.editor.path /usr/bin/kate
   * Now use Ctrl+U or Page Source to open the editor.

Now by source view Firefox trigger to open editor.