I used a cut-n-paste sample that caused problems and https://edg2s.github.io/content-editable-sandbox/ to produce a short snippet that results in an error if pasted in IE or FF. The snippet is:
<font></font><ul><font> </font><li>dummy text</li></ul>
Removing either pair of font tags or the space between the second pair removes the error.
FF and IE both point to ve.dm.Node.static.cloneElement and indicate that clone is null and/or does not have an "internal" property.
The root of the problem is that we convert <ul><p>Foo</p></ul> to [ list, paragraph, F, o, o, /paragraph, /list ] which violates list's childNodeType constraint (it should only contain listItems). We should do something more sensible.
The content I got was generated by Outlook when pasting into Internet Explorer.
Note that what I'm getting is <ul> <font> (SPACE) </font> <li> content </li> <font> (SPACE) </font> <li> content </li> <font> (SPACE) </font> </ul>
It may be that "<font> (SPACE) <font>" is seen as essentially null. I wouldn't want additional <li>'s created, but perhaps moving the <font> tags inside the <li> would be fine if you're worried about losing content. Alternatively, just dropping "<font> (SPACE) </font>" wouldn't offend me.
At the weekly triage meeting on 2015-07-07 we decided that this was important but not urgent to do this quarter.