As a user, rather than a developer, I have 4 extensions which I wouldn't be without.

Adblock Plus does what it says, and is easy to customise.

IE Tab is so easy for those badly written sites that display correctly only in IE. One click switches the page rendering engine for that page only.

I use Real Player, and the Real Player Browser Record plugin identifies all recordable content on a page, and a single click downloads it. Works fine with YouTube and many other sites.

Finally, I use SnagIt for screen capture, and the SnagIt Extension puts a toolbar in Firefox which accesses all the basic functions of SnagIt.

As far as pdf viewer is concerned, I haven't found a Firefox extension other than Acrobat yet. I use FoxIt for most pdf reading, but have to suffer the slower Acrobat within Firefox.

Phil