Why not ask the person who told you such things exist to show you how he did it? After all, it's not yet April 1.
Kees
Certain forum uses some sort of a modification which allows collapsing certain categories but it doesn't permanently remember which categories I have collapsed. I was told to look for an addon which will create a cookie to remember permanently which categories have been collapsed. I don't know what I should type in the site with firefox addons or in Google, so I'd appreciate if you can tell me what to look for or even better - post links to such addons.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic