After upgrading to AVG 2012 summer edition firefox started to use iSearch to return results rather than google. The problem with iSearch is that it took too long to return the results. It seems to me that when you type your search string in the address bar (URL bar), it is taken to iSearch which in turn goes to google and then show you the result. Since it is from AVG, I will not be surprised if it does some link checks in between. If this is the case and you are a security enthusiast then do not do anything else cause it sounds secure.
However, if you find it annoying, here is what I did to get the search hit google just like old days.
1. Open a new tab and type the following (no space in between) and hit enter or the return key:
about:config
2. A warning message shows up. Keep the box checked and click on the button "I will be careful I promise".
3. In the filter text enter the following:
keyword.URL
4. You should see the same text under the 'Preference Name' column.
5. Right click on the 'keyword.URL' and select 'Modify'.
6. In the 'Enter the String value' window, remove the entire content. You can save the string value in a notepad
somewhere if you want to return to this feature.
7. Click 'Ok'.
That's it! This should take you back to Google in place of isearch.
There are some other options mentioned over the net like removing the AVG search option, Manage search engine, Restore default etc. Those did not work for me.
NOTE: Some users have reported that they needed to restart their machines. I don't see if that would be necessary but you never know!
Hope you find it helpful!