I forgot to mention the alert that pops up says "before write". Note also there's a typo in the console line.
Can you attach a complete web page that shows the problem?
Created attachment 494459 [details] repro example Attachment is a simple web page and script which demonstrates the problem. Works fine in Firefox 3.6.13 but not in Firefox 4.0b7. Thanks.
Hmm. I see the issue in b7, but not on trunk. Looks like this got fixed somewhere in this range: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=ad227939db82&tochange=789f0f85f75a
NOTE: this now appears to have been fixed in the released version of 4.0b8.