From the liberal use of $, I assume you are using jQuery. Which version are you using?

I'm using jquery-1.11.1.min.js

I was going to try to build a test case in http://jsfiddle.net/ but actually this may be hard to test without a general idea of the length of your content. Is there any way you can post a page demonstrating the problem?

http://www.greyhoundpets.com/greyhounds.php

Near the top of the page, you'll see a "Select name" pull-down list. Select a name and click on the "Click to find" button. This should scroll to the requested name, but does not.

BTW, if you scroll down the page you will see a "Top" button appear near the bottom right. This button also uses the jQuery scrollTop function and it still works in FF36.

If I edit your button's onclick handler to your actual script, it works:

<input onclick="var e = document.getElementById('findname'); var x = e.options[e.selectedIndex].value; var $target = $('#'+ x); var targetOffset = $target.offset().top; $('html, body').animate({scrollTop: targetOffset-15}, 900);" class="switchbutton3" value="Click to find" type="button">

That makes me think that Firefox might be interpreting

onclick="scrollTo()"

to refer to the native window.scrollTo() function instead of your function. I don't know why that wasn't a problem in the past, but perhaps if you rename your function to something like scrollToName() that will resolve it?

That was exactly the problem - I renamed the function as you suggested and all is working again now. As you say, it's odd that this didn't appear to be an issue until FF36. Many thanks for taking the trouble to help me with this.