A different answer might be that it does so much more that a slowdown is part and parcel of doing all that's necessary in a modern browser.
Firefox 1.0's USP at a time when browsers were bloated was its lack of bloat. IMO thats less of an issue now.
In this case Firefox has even managed to outperform Moore's law. It managed to become slower in a faster rate than computers get faster.
My argument is that I'm not necessarily the main target demographic any more, Firefox 3.6 is fast enough and has the features that Joe average wants.
Phoenix was fast. I liked that. I should go try to compile it and run it on W7 and see what it does on modern websites.
Speed and ability to render correctly are my killer features for browsers.
1. Sync slows down the interface of the application a lot. There should be an option to completely disable Sync. Or Sync should have been an add-on installed by default. If I want to sync only the bookmarks, do just that, nothing else. Sync should also be activated only when I save a new bookmark, not every now and then.
2. SQLite slows down the application and complicates things with those default smart folders. You can't even create a smart folder for bookmarks. Mozilla should have write a faster database for bookmarks and history — they are just soem lines of text after all.
3. Live Bookmarks are useless now since the RSS icon was removed. It was useless before. I think by removing the code for Live Bookmarks and treating bookmarks and history in the same way will make things work faster.
4. Tab Groups also slows down the interface. It is useless since you can manage multiple tab groups by opening new windows.
5. Moving the status bar in the address bar was a bad idea. It complicates the user experience. Status bar should be shown only when something new happens — a download stars, a new page is loading, etc.
I don't know what happens under the hood, but Firefox is heading in the wrong direction.
(2) Places (the bookmarks database) is now on asynchronous APIs, which means that, unless you're using esoteric features of bookmarks, they never block the UI thread. SQLite is also now in WAL mode. Smart folders are still slow as I understand it, and there's a plan to revamp and/or remove them.
(3) Bookmarks and history have been treated the same way since Firefox 3 (Places). There is talk of revamping and/or removing Live Bookmarks as well.
(4) Tab Groups is now secondary UI (the icon is gone by default). It is never loaded until you open it.
(5) The status bar is no longer in the address bar.