We are aware about this, and we had prepared scheduled release plans to cover Firefox's releases with already compatible AVG add-on, but our schedule was delayed by other discussions and additional verification. Because of this also releases of compatible AVG add-on are delayed.
Please keep in mind that this add-on (link to FAQ) evaluates search results and typed links based on internal database (updated daily like virus database updates). Main protection from exploits in source codes of visited websites is LinkScanner's Surf-Shield component, which is checking whole HTTP communication and is not bound with used browser (or it's version).
In addition, we are already running our AVG Secure Search (powered by Google) at http://isearch.avg.com, which warns about malicious websites in search results based on online database and without need to have AVG installed (evaluation is being made on server side).