I ran into a series of strange errors while using the FireFox Error Console to troubleshoot a JavaScript problem. The errors were unrelated to the script I was working on, which led me to start hunting for the cause of the problem.

For programmers who sometimes write scripts using JavaScript, troubleshooting these scripts can be difficult because you frequently cannot see where a problem lies. The FireFox error console is an invaluable tool to use for troubleshooting JavaScript, CSS and a few other problems. It pinpoints errors in the code and can help you to identify most problem areas very quickly. You will find the Error Console under the Tools menu in FireFox.

When I opened up the Error Console this morning, I found a strange error message that was repeating in the Error Console window about every three seconds. The real problem was that the code identified as causing the problem did not exist in my script or any other script that I have worked on for several years. The error was identified as a missing closing tag on a refresh meta tag.

mismatched tag. Expected: </meta>.
<meta http-equiv="refresh" content="0; url=&#39;http://mail.google.com/mail/signup&#39;"></head>

The refresh meta tag is an html-level method for either refreshing a page or forwarding a user to another page. The problem with this error is that even if the line of code existed in my script, it should not have generated an error because the meta tag identified as the source of the problem does use valid HTML code, but would require a closing tag if it were XHTML. But there is no such closing tag for meta tags as the one the error indicates is missing. The proper method for closing meta tags in XHTML is to use a slash at the end of the tag.

My next thought was that perhaps our server picked us a strange virus or malware. I tried several other web sites and received the same error with each of them, so the problem was not related to our server.

This is a little blurry, but here is what the error messages look like:

The fact that the messages were automatically repeating every three seconds rendered the Error Console useless, so the problem needed to be resolved. Notice that the script appears to be trying to forward a user to the Gmail login page. While we do use GMail, I don’t have any links or bookmarks set up to the GMail login page. However, the Google Toolbar was enabled in my FireFox browser. I disabled the Google Toolbar and the error messages disappeared.

The versions of both the toolbar and FireFox that I am using are current.

In the past I have occasionally seen some strange error messages that appear in the Error Console. Most of the time they are for CSS errors that do not exist in my scripts. Those were random and rare, but this issue was so persistent that I could no longer use the Error Console.

If you run into some strange errors that you cannot identify in your code while using the FireFox Error Console, try disabling any FireFox add-ons one-by-one to see if that resolves the problem.