works in firefox and IE7 in version 2.5.1 but now in 2.6.1 IE7 throws up the error but firefox is fine…
Yes. It appears to be a known Internet Explorer issue. Not Firefox, and it still sounds like you might have a plugin compatibility issue. Rule that out first, before tackling the issue of whether or not it is related to a change in WordPress code. I say that based on the following;
A) I am assuming the fckeditor you use came in the form of a plugin.
B) Not knowing which fckeditor plugin you chose to install, (and I am sure there must be many), or if that version is compatible with WordPress 2.6, and not being able to look for myself because your server prevents browsing of the directory, ?? this is just a blind stab in the dark… but it might provide insight.
https://code.google.com/p/fckeditor-for-wordpress/downloads/list
Internet Explorer cannot open the Internet site” error
Be sure to view the fckeditor_error.gif image located at the bottom of that post. Look familiar?
The fact that the plugin versions appear to be different for WordPress versions 2.5 and 2.6 (if I am reading the version information correctly), suggests that there were some known issues. Perhaps this is one of them. I don’t really know.
Not a solution, but it’s a start in the right direction.
Best wishes.