Firefox will popup an error in a number of situations. Most often this is caused by a misbehaving extension.
Before going through complicated troubleshooting steps, try disabling these extensions:
Click the menu button
and choose Add-ons. The Add-ons Manager tab will open.
If the steps above didn't solve the problem, try these articles:
I had discussions with Cheng and Bram about this. In the case where a topic page was narrowly focused as this one is, it's good to add a solution when appropriate.