Lately many of Joomla 1.5 site owners have been experiencing weird Java script errors when tried to use their site with FireFox browser. Receiving lots of request almost simultaneously the first reaction was that some software update caused the problem.
Turned to be true, Firefox 18 and the version of MooTools Java Script library that ships with Joomla 1.5 are not compatible. So, sites NOT using the MooTools are safe, and also Joomla 1.7+ sites (including all Joomla 2.5 and Joomla 3.0 versions) are also safe.
Another reason to upgrade 
.
Okay, upgrade (more precisely migration) can be weeks away, is not a simple process, so what can be done? To fix this issue you will need to open up plugins/system/mtupgrade/mootools.js and paste the following snippet of code at the very top of the document (before the MooTools declaration).
String.prototype.contains = function(string, separator){
return (separator) ? (separator + this + separator).indexOf(separator + string + separator) > -1 : String(this).indexOf(string) > -1;
};
This should fix the problems you have been experiencing. To see if this is what caused your problems accessing your site you can do a simple test - try to access it with Chrome, IE, Opera or any other browser not build based on same rendering engine. The problem is Firefox 18 specific. Be aware, that the fix qualifies as a core hack. Be careful, make backups, and use the solution on your own risk.
