When I open the error console I get hundreds of lines of error messages like this:
Component is not defined
chrome://fdm_fftext/content/fdm_ffcxt.js

And then all dissaper automaticly.

The JavaScript errors of my web pages are also somewhere in those messages but it is impossible to find them, since they too dissapear with all those few hundred lines.

Anyone have any solution to this?

Here is a screenie:

That seems like an error with one of your addons. Uninstall all the lately installed ones and re-install them one by one until the error is back. That should give you some clarity about the cause of the error.

Your firefox has .js missing. Maybe it's a add-on...
Try what amygdela

Thanks guys,
amygdela was right, it was FreeDownloadManager extension.

You're welcome! Glad I could help.