Subdocument frames are not collapsed in Firefox

Environment

ABP 3.6.3.2396
Firefox 51.0 / Windows 10 1809
Firefox 69.0.1 / Windows 10 1809

How to reproduce

1. Navigate to the subdocument test page.
2. Add the filter.
3. Refresh the page.

Observed behavior

The subdocument is blocked but the frame is not collapsed.

Expected behavior

Frame to collapse as it does in Chrome / other browsers.

Hints for testers

In order to address this issue we had to refactor the whole element collapsing mechanism, and adapt the filter composer code (i.e. the "Block element" dialog) as well. Please make sure to thoroughly retest these features.

See also

The previous ticket (#6917) on Trac.
The related Firefox ticket (https://bugzilla.mozilla.org/show_bug.cgi?id=1488226) on Bugzilla.