Triggers rarely-used blocking feature to protect users from attack
http://www.computerworld.com/s/article/9139518/Mozilla_blocks_Microsoft_s_sneaky_Firefox_plug_in?taxonomyId=125
Please read this topic. 
Especially this post:
http://forum.avast.com/index.php?topic=49871.msg422873#msg422873
End result is its not a threat if the system is up to date.
Sorry, I guess my post was rather lukewarm. 
Hi Rumpel,
Again Mozilla nog longer blocks it, because MS stated it has patched the hole and there is no longer a security risk for Fx: http://blog.mozilla.com/security/2009/10/16/net-framework-assistant-blocked-to-disarm-security-vulnerability/
Update (Sunday Oct 18, 6:30pm PDT): Microsoft has now confirmed that the Framework Assistant add-on is not a vector for this attack, and we have removed the entry from the blocklist. We are also working on a mechanism to allow Firefox users to re-enable the WPF plugin ahead of its eventual removal from the blocklist. For more information, see Mike Shaver’s latest blog post.
The show must go on, and a new news item for inside this thread,
polonus
Thank you for the well-sorted info, polonus …but I’m afraid to feel we might be making mess with two threads… cf http://forum.avast.com/index.php?topic=49871.msg422809#msg422809
Hi Rumpel,
Then add the link to my posting above there as well, so all in these threads will get the latest on this,
pol
Hi, polonus, thank you for your teouble.
I wonder if some still care but I happened to spot that Calender of Updates publicized how to remove the unwanted FF plugin.
not anymore on Firefox block list after MS released a patch …doesn’t change anything for me, I still don’t want the freaking plugin ;D
http://www.pcworld.com/article/173877/mozilla_unblocks_microsoft_addon_for_firefox.html
for me, I have not installed .net framework. i do not install any software which requires .net, safe here.
nmb
My main PC is free from Net Framework but my family members are not…although they only have portable version of FF, which is free from the plug-in. So, please take it for someone who cares.
That’s interesting, I am sure I remember having to disable it in my portable FF…
??? That’s odd. All copies of FF Portable on our computers didn’t have the plug-in despite the fact that .Net Framework 3.5 is installed on some of them and that all FFP copies are under program folders, which is why I thought FFP was not affected.