In the case of google.com.au instead of google.com, there is no real difference. Those are just mirrors of the same webpage.

If you want to use a different search engine (for example searx.me), those engines often provide an addon under addons.mozilla.org or an "Add Search Engine" link somewhere on their main webpage.

There are Google search engines at http://mycroftproject.com/google-search-plugins.html including google.com.au

Thanks mate it's more a case if wanting to filter search results on country sometimes. If I lave it as .com it doesn't filer on the country that I want. Also Firefox used to choose the country I was in ie .au etc automatically -now it doesn't. But thanks for your help found the search engine I wanted.