I knew you'd use Photoshop as an example, does that tell you anything? If you Google around for people arguing that 32-bit userspace is fine, and no-one needs to worry about running out of address space and so on, the first things you run into are Adobe Photoshop developer blogs.
They've been making this argument ever since 64-bit Windows became available to ordinary mortals. You can find plenty of made up numbers about how it would actually be slower for most purposes, and certainly wouldn't get any faster until you were loading absolutely huge images with vast numbers of layers...
... and then Adobe announced the 64-bit Photoshop and suddenly the story changed overnight. 64-bit was going to be faster for even moderately sized images, a lot of existing features would see a speed-up, and so on.
This is called /spin/ - Adobe used to tell Photoshop users they didn't want unlimited Undo History, it was a crutch for people who didn't use their image editing software the right way, and then one day Photoshop implemented it and suddenly Adobe couldn't say enough about how great it was. They're trying to avoid the Obsourne effect, I get that, but the net result is that they spent a lot of time misleading customers. Why believe anything they say?
Adobe ended up in the mess with Photoshop, partly, because of a lack of abstraction. Many Photoshop users have 3rd party plug-ins. The plug-ins run "in memory" so if you upgrade to 64-bit you need some tremendous kludge or else you can't run any old plug-ins. A bit of design foresight would have been good right there.
You're mistaken about PC video games, it's nothing to do with 3D or Windows. It's all about DOS extenders. The DOS extender was supplied with your tool chain, effectively part of the run time. Despite DOS being 16-bit the "extender" would push the CPU into 32-bit flat mode, so all your actual development would be in 32-bit. DOS extenders were fairly rare when Doom had one, but by the time Quake arrived in 1996 everything in the PC section of your games store would be 32-bit code run by a DOS extender. Even simple point-and-click adventure games, which could have been coded for 16-bit DOS with overlays or similar tricks weren't bothering, 32-bit development meant less headaches for developers, which meant shorter time to market.
I'm not really sure what to make of your claims about DOM and so on. If web browsers actually need billions of DOM objects, URIs, images etc. then isn't it a surprise that they've ever worked on 32-bit machines? Or on the other hand, if as seems more likely they have a few million DOM objects, a few million URIs, and a few thousand images, then 32-bit handles are fine, and will be long after 32-bit memory isn't nearly enough. When I said most programs don't need lots of pointers, I meant that explicitly, and you don't seem to have understood that. /Choosing/ to use pointers everywhere you can is a design decision, and not automatically a good one.
Finally, I must say that although Firefox feels sluggish everywhere, it doesn't seem especially more so on my 64-bit machines. "Significance" is a tricky term, medical researchers speak of both "statistical significance" and "clinical significance" e.g. maybe you can show that your new drug has a statistically significant effect on lower back pain, but then when doctors look at the size of the effect it's equivalent to one person saying they feel a little better for every million people treated - well, that's not clinically significant, your drug is useless. Negative effects from increased native word size need to be characterised in terms of what users would actually notice. If the user won't notice, we shouldn't care.