this is probably caused by an addon / userscript which is doing some operations/modifications on the page while google redirects you do a search result. you can try to run firefox in Diagnose Firefox issues using Troubleshoot Mode and see if the error still occurs there. if not disable all your addons & reenable them one in order to find out which one is causing it exactly (you'll likely have to do a restart of the browser after each one).

Thanks. I disabled all my addons, every single one, but it has made no difference:-(

Google is always busy updating things, and Firefox can sometimes get files out of sync. When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Offline Storage : "Clear Now"

2. If needed, delete the site's cookies here

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?