I have 5.12 installed, and everything seems to be fine EXCEPT:
The default view of the compose/edit Body field is Rich Text and WYSIWYG. If I click the HTML button, it switches to HTML view... but clicking back on WYSIWYG mode doesn't work... the view doesn't change and the article is "stuck" in HTML view.
In the past, I have run into similar issues when neglecting to use a leading "www" (or appropriate subdomain) in the URL of the installation, but that is not the case here.
This is my first time installing 5.12... I haven't found any articles, but I'm wondering if this a known issue or if anyone has suggestions for how to fix it. Even though we're still in development, the client has already noticed it and been frustrated while adding content.
Reported on Movable Type 5

More info: I have confirmed that this is an issue with Firefox 6.0.1. However, Chrome and Safari do not have the same issue. Any ideas for how to make it work in Firefox too?
Hi,
You should know that Six Apart KK is trying to address browser compatibility issues.
There are some issues with IE v9.0 and FF (some versions).
See Jun's feedback on a similar thread at:
http://forums.movabletype.org/2011/12/edit-templates-wfirefox-8-hangs-mt-5.html#comment-212897
Kind Regards,
Mihai Bocsaru
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Thanks, Mihai! I see... It feels like Firefox has been regressing in quality the past couple of versions, so this is not surprising. Hope there is an update soon to address the issues (and thank goodness I don't have to worry about IE9).
You're welcome! Indeed FF is on a descending trend.
I've stopped using it more than 6 months ago because of all kind of issues spreading from speed to flash player conflicts.
At the moment I'm using Chrome, because it's a lot faster, even if not as versatile as FF or even the old, buggy IE.