"Delete" action from context menu doesn't work for multiple paragraphs

[Softvision-CristinaBadescu]

[Notes]:

• This issue is neither reproducible if you use the "Delete", nor the "Backspace" keys.
• This issue is not reproducible when deleting a single paragraph.
• This issue is not reproducible if you select multiple words from multiple paragraphs using "Ctrl"/"Cmd" and click.

[Affected versions]:

• Firefox 57 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile with the latest "Firefox Notes" add-on version (1.9.0b2 build with CKeditor) installed.
• Have at least two written paragraphs in "Notes" sidebar.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Select the paragraphs using the mouse.
3. Right click and choose "Delete" option from the context menu.
4. Observe the behavior.

[Expected result]:

• The paragraphs are deleted.

[Actual result]:

• Nothing happens.

[Additional notes]:

• This issue is also reproducible when trying to delete different words from both paragraphs.
• Here is a screen recording of the issue:
  

[vladikoff]

@Natim I wonder where this context menu is coming from, ckeditor?

[Natim]

Firefox

[vladikoff]

CKEditor bug it seems: ckeditor/ckeditor5#684

[Natim]

This is handled in Firefox with the two following bugs:

Bugzilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1379584
implement beforeInput bug: https://bugzilla.mozilla.org/show_bug.cgi?id=970802