Boy this was a tough one.
Back when I stumbled across and installed my favorite plugins they didn't seem so difficult to find. I guess the "stumbled across" part is what made it easier.
However, there is another factor now, massive competition in the plugin world. Doing a search for plugins turns up a zillion possibilities where a short while ago it was just a few. So now, I need to wade my way through a fair number of "maybe this" and "maybe that" to find the plugin that is the one I used.
Might sound like I use a ton of plugins. Not so. I just like the few I use.
Web Developer 1.1.6
https://addons.mozilla.org/en-US/firefox/addon/60
Html Validator 0.8.5.2
https://addons.mozilla.org/en-US/firefox/addon/249
IE Tab 1.5.20080823
https://addons.mozilla.org/en-US/firefox/addon/1419
HttpFox 0.8.2
https://addons.mozilla.org/en-US/firefox/addon/6647
I've listed them here. So that maybe next time I go through this maybe I won't have to hunt so hard. I'll just read the above and "ta-da" there they'll be.