This sounds like you had the iframe version enabled. Can you confirm if you had the navigable table of contents, template editing or syntax highlighting turned on?

admin wrote:

(In reply to comment #1)

This sounds like you had the iframe version enabled. Can you confirm if you had
the navigable table of contents, template editing or syntax highlighting turned
on?

In LocalSettings I have indeed:
$wgWikiEditorModules['toolbar']['global'] = true;
$wgWikiEditorModules['toc']['global'] = true;

So, your hint is to turn table of contents off, since that somehow enables the iframe version?

Yes, the iframe is unfortunately quite unstable at this time, and may never get the love it will need to become stable. We spent a rediculous amount of time on nailing down all the edge cases to make it work across all modern browsers and were met with a myriad of problems that eventually led us to accept that we either did not have smart enough people working on the problem (I was one of them :P), did not have enough people working on the problem (there were about 4 of us) or don't care enough about the problem to continue working on it.

admin wrote:

I understand: there were four loving people who were not too keen on caring about their work? Do I get that right?

Honestly, thanks for pointing me to the right direction, although the problem is not to be solved. And sad to hear that the work spent on stabilizing still leads to some problems, at least for some users. I turned the toc off, now the browser doesn't crash anymore. I'm lowering priority to normal.

But two problems still exist. The first is that when pasting the editor adds lots of white-spaces, about 2 per line. And the second is that all the breaks within the pasted text get lost, no matter what origin it comes from (notepad++, wiki). When pasted into the WikiEditor, there will be only one (huge) section.

If you had another useful hint to get along with that, I would be grateful.

Where are you copying the content from? We built a copy-paste matrix at one point, identifying the behavior of copying from various sources on various platforms to various browsers. There were lots of issues around copying from rich text editors like Wordpad, Microsoft Word, etc. and into some but not all browsers. I don't recall firefox being the most stubborn of these, but you may have hit on a problem combination. Also, pasting is much more reliable with the Navigable Table of Contents module turned off (with the textarea rather than iframe implementation).

Trevor was looking at this for you, but you didn't respond. I'm closing it, but if you're still interested, please reopen this.