Add-ons that would be great as restartless add-ons, since you only use them occasionally
Add-ons essential for your everyday online privacy. You also need HTTPS Everywhere https://www.eff.org/https-everywhere and https://startpage.com. For special privacy, use Torbrowser instead.
Add-ons useful for shopping (ie. finding cheaper prices).
Themed personas I like to install on other people's computers.
A collection of lightly-coloured, clean (not-too-busy) personas I use (in rotation with the Personas Rotator add-on).
Extensions which are great but use too much CPU - if you have some über six-core machine then they might be worth using.
Some search engines don't provide suggestions and so are no better than bookmarks - unlike all of these!
Add-ons which fix bugs, improve security, or add functionality that should be included in Firefox by default. See also my collections "privacy essentials", "functionality" and "newbie-suitable".
My favourite add-ons, purely for aesthetics. I also recommend my collections "bugfixes", "functionality" and "privacy essentials".
My favourite add-ons to extend Firefox's functionality, with minimal slowdown. I also recommend my collections "bugfixes", "privacy essentials", "pretty" and "newbie-suitable".