My Linux box which has similar horsepower to yours was also starting to feel a bit sluggish. I don't think there was anything wrong with the OS or with the browser(s), but with the web content which grows ever more bloated with both flash(y) ads and even content that has way too much. I love Mozilla and root for their success. But with so many people now switching from IE (which still sucks too) reliability is more important than ever. I've been using Chrome more and more and found it to be faster, but also found it to fail on some pages that Firefox handled fine. I've also upgraded to a low-end Apple machine for my primary desktop and found *all three* browsers available there, Chrome, Firefox, and Safari, fail to render some pages properly, or at all. I'm getting faster page loads, due to a faster machine, and bumped up bandwidth (Verizon has a free upgrade for a year offer going on). It *is* a conspiracy, there is no doubt about that. I had gone so many years without upgrading hardware (and my old hardware was purchased used) that finally getting "state of the art" hardware feels worth it (for a change). But the vested interest will do everything in their power (going to solid state disks, 4 core processors, and other gimmicks) to force you to buy new hardware to make up for their lack of concern about performance or reliability issues with their server side operations.
Some of these pages from "mainstream" sources depend on not just graphics and text, but scripts from a dozen different servers, and it's not just ads, but also measurement services, services that underline certain words and provide pop-up definitions (a stupid idea IMHO) and other such "features" that makes "joe programmer" at the Gazette look more productive than he actually is. There is a house of cards being built here among failing media companies. A house where everyone can point the finger of blame a someone else when it's convenient, or take credit all to themselves when everything is working. Unfortunately the "everything is working" scenario is proving to be more and more illusive.
In the "good old days" browsers could load web page text fairly early and put placeholders in for graphics that might or might not eventually load. At least you could start reading in the mean time.
Today's pages often start out by running a script which then fetches the actual content, along with doing a dozen other things. I think if the authors of these scripts were really good at what they do (which they are not) they would also make sure that they produce something useful, even if everything doesn't go quite as planed. They don't do this though, and there is no way for the browser to "read the minds" of the script writers.
I don't know that there is a way out of this other than for users to let publishers know that if they are going to keep publishing content that isn't readable you won't be patronizing them any longer. Letting them know this publicly (in comments if you can get to that point) may help more than a "letter to the editor". I've given several of the MSM outlets grief over this and have seen gradual improvements, whether as a result of my comments or not I can't say.
By the way I've been using Debian for many years and never saw the need to go to a more "user friendly" distro such as Ubuntu. I always had to tinker with my initial installation to get things like video and sound to work right, but that was mostly adjusting control files, not actually replacing drivers etc. Once I got a Debian version running I almost never had upgrade issues, except when I brought them on myself by doing a fresh install from time to time. The latest Debian I installed a year or so ago needed no such tinkering for the first time. Everything just worked!, and still does. It's nice to have a still-working backup machine in case the primary fails, and there is a good chance that at some point I'll be running Debian instead of OS X on this machine as there are many things I prefer about Debian (or Linux in general) to the one-size-fits-all approach of Apple.