Right, just installed GreaseMonkey and the script in Firefox and tried it. The problem definitely seems to be to do with Firefox or GreaseMonkey as I'm getting it too.
As Hyper suggested, copying and pasting it works whereas the script doesn't. So presumably there is some kind of conflict in how the script is run or with something in GreaseMonkey. I don't really know much about the permissions aspect of userscripts so I'll have to do a little research but now that I can replicate the error hopefully I can fix it.
But that nodeType error does seem to be part of the problem, so at least I have something to work on.

I'm sad now; the script stopped functioning for me too now. As HSL indicated, it clears the posts on the screen, then just sits there seemingly doing nothing. It started sometime this morning, I think with the latest Greasemonkey update.

I've got Firefox 39.0.

Yeah, preliminary results are suggesting it's something to do with Greasemonkey. I did wonder if there had just been an update (I use Tampermonkey in Chrome) so that does make sense. Looks like something has buggered around with the permissions and it doesn't like the AJAX any more... Hopefully I can find a way to fix it.

Ok.. I've found where the problem is, now I just need to find why it's a problem and what I can do to fix it. I'm working tomorrow so it might take a few days to get it right. But I will try and put up a temporary fix tomorrow if I get a chance.

Great. Thank you.
I'm already beginning to experience withdrawal symptoms.
This forum search thingy that GOG offers is just too weak.

P.S. Take your time...

You can probably make the temporary fix yourself if you want. The problem seems to have arisen because I moved the actual post filtering bit into its own function (because it was used in two different places and it would be easier to maintain that way)
You can just copy and paste the contents of the function threadFilter and use it to replace the calls to the function (you neex to fix the variables too)

What I can't figure out is why the forum one seems to work but not the thread one, because they're both very similar. The only thing I can think of is it's a specific element that I access that's causing the permission errors..
But I'm getting there and hopefully it's fixable, if not it will just mean reverting to the original method.

Right. I'm a complete moron. Found the error, turns out I had missed one of the variables when I took the code out and made it a function... *facepalm*
Anyway, should all work again now.
I've also added some debug stuff in (probably far more than required) but it's set to off by default.

Let me know if anyone is still having problems.

How do we get the update manually? I'd love to have it, but I don't see an option to the effect of "Check for Updates".

Yay! Auto-update kicked in and the script seems to be working again. Good job, Adalia!

If it's Greasemonkey then you can force an update by right clicking and selecting update.
Glad it's working again :)

Where should I right click? I tried the icon, but I didn't get an "Update" option from that menu.

If you click the arrow beside the Greasemonkey icon in the bar and click Manage Userscripts it should give you a list of installed scripts. Right click on the script in question and click Find Updates (or Force Find Updates if you've made any changes to the script yourself)

Ah ha! I didn't pursue the "Manage User Scripts" selection. Thanks!

Life is good again. Thank you, adaliabooks. +lots.