Shit started with the "smart" URL bar in Firefox 3. It behaved completely differently to the Firefox 2 URL bar, and there was no getting back to the original behavior. Which wouldn't be bad, except the new URL bar was became incredibly slow on certain slow software configurations (largely dependent on what file system was used) after a couple of months of use.
There has been many more changes like this through the years. The official mozilla policy seems to be to never offer ways of fixing a slow or frustrating user experience ("switch to tab" in newer firefox, how I despise you), and hand all that to extension developers. Except, they're rarely able to actually fix the problem.