Actually, Tenfourfox today is much faster than older versions or even Firefox 3.6. It does take longer to start up, but once it's running, it should load the same websites a lot faster. On the other hand, websites are more demanding today than they used to be five or ten years ago, and ity's not always possible to balance this out with a faster browser.
You should go to Help>Restart with Add-ons disabled and then try to load some websites. If they're faster, restart the browser again and go to Tools>Add-ons, then disable all add-ons you don't really need. Adblock for example slows down the browser considerably, I replaced it with uBlock Origin, which is much faster.
Also, a large history can slow the browser's navigation down.
Things like a profile reset do help to clean out accumulated cruft,
and you can still keep your bookmarks and certain other settings.
See below right ("How to reset your profile").