Saulkar said:
Is it because the websites are so much more demanding or did Firefox get worse?!
A little bit of column A, a little bit of column B.
Websites have definitely grown more complex and demanding over the years. In some cases you can definitely argue it's "convoluted". Previously, you'd be perfectly fine with a simple HTML page but nowadays JavaScript is pretty much universal around the web, so each website will require more computing power.
Firefox did definitely change, too. You could call it "getting worse" or you could argue it was "evolution" and in reality it's again column A and column B. It's not only that websites change - web standards also do. There is more stuff to support than there was 10 years ago - we have an additional major revision of CSS (CSS 3), technically two versions of JavaScript (ES5 and ES6. ES4 wasn't around a decade ago but it didn't the light of day because it got scrapped), as well as one major revision of HTML (HTML 5). Moreover, there is just way more stuff that browsers do, nowadays - including media playback and security. Everything on the web gets to be much more complex and modern browsers need to be able to handle it.
That's not to say that Firefox has only been improving, of course - for example, the browser has been infamous for its memory problems. In all fairness, it HAS been improved quite a lot, but it's still not an issue. There has been other stuff, as well, like changing the browser modding API and the like that would change how it works.
So, all in all, it's changes everywhere.