This is post is now quite old and the the information it contains may be out of date or innacurate.
If your firefox address bar is getting a bit slow and laggy as it tries to find suggestions, I found one single about:config tweak totally sorted it out for me.
browser.urlbar.search.timeout = 100
browser.urlbar.search.timeout = 10
To do this, open a new tab and type about:config
Then agree to the thing that says you know what you are doing and in the filter bar at the top type:
you can then double click on the value (set to 100 by default) and change it to whatever you want. I tried 10 and that seems to work great for me, but you might want to experiment.