You can also edit your userContent.css file directly, without downloading AdBlock, etc -- and you don't need to upgrade your older version of Firefox to do it. It's probably more complicated then downloading the plug-ins, though. (Although it may be faster -- on this old computer I'm using I found AdBlock quite slow when I first tried using it to block the old benner ads.)
Credit for this solution totally belongs to VampiricMushroom on the IDB forums and Olivia on the Neocolours forums, I know jack-all about CSS:
To find your userContent.css file:
On my XP machine the userContent.css file is in the following folder:
C:\Documents and Settings\mynamehere\Application Data\Mozilla\Firefox\Profiles\eausb527.default\chrome\
Once you get into ..\Firefox\Profiles\ I think you just keep going down until you get to the chrome folder.
Obviously the folder name will be your name, not "mynamehere." I think the funny-name directory may be different on different computers as well, but that's OK, it's the only subfolder in the "Profiles" folder, so just go into whatever's there.
Note that the userContent.css file might not be there already. I didn't have one until I created it. It goes in the "chrome" folder. Just make it in Notepad and save it there as "userContent.css".
This is the complete content of my userContent.css file. The first line blocks the small side ad. The second line blocks the big ad (although actually you don't really need it if you have the last line, I left it in there for the sake of completion). The third line blocks the NQII title banner because I want the extra space when I play NQII. You can leave it out or add a wildcard so it blocks all the top title banners as you please. The last line gets rid of the white space.
Code:
embed[type="application/x-shockwave-flash"][width="120"][height="60"],
embed[type="application/x-shockwave-flash"][width="728"][height="90"] { display: none ! important }
img[src="http://images.neopets.com/new_headers/games/neoquest2.gif"]{ display: none !important }
div[id="ban"] {display: none !important;}
Note that the user who I first saw post this fix on the IDB forums (VampiricMushroom) mentioned that it ought to be possible to do something similar in any browser that accepts user-defined style sheets, which I think includes IE. However, the commands may not be the same and I have no idea how to go about it, being generally clueless about css. I'm told by fellow Neocolours users that it works in Mozilla and Safari, if you can find the right place to put the file, but I haven't tried it myself.
I've not noticed problems browsing other non-Neopets sites with this so far, but I can't make promises. :-)
(Edited to fix some typos.)