this.xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
It turns out that tw-sack.js was not the only file affected – also post.php and wp-gears.js showed this error.
I googled this – but I found nothing helpful.
The error doesn’t seem to affect the functionality of my blog, but I hate these red-and-white Firebug Xs!
After rebooting and clearing the cache etc., everything seems to work now – the error hasn’t appeared since.
Is this a Fire”bug”? Or is it related to the recent Windows Update for October? Or is it just because there were some failures or incompletely loaded files within the cache, and clearing it was all there was needed?
[Update from 2008-11-20]
Hmm… I just recently installed FF 3.0.4 and Firebug 1.3.0b3 – and the error seems to have disappeared.
It’s back! I changed nothing! This is some odd behaviour, I must admit…