Forum: at creating/edit new threads or post/comment followin error is very common! also if you click on new threads at the right side you get: Wiki:
Also, when the error happens press "Ctrl + Shift + K" to see the Web Console and there might be some additional errors. + Also what you can do is show the source page while the error is up and there might be some more detailed info about the error. Press "Ctrl + U" to show that. But post things into code tag, because they are huge. (keyboard shortcuts are for firefox/waterfox, not sure if they are the same in other browsers)
If you're going to be doing debugging in Firefox, you might as well get Firebug. Then you just press F12 like in Chrome.