Is there a way to disable certain addons while in private mode? There are times that I need a Firefox window open with certain addons disabled simultaneously with another window where they are enabled?
I am not sure if this is possible if you intend on using the same profile for two addon criteria. Even if you try two profiles, I doubt they can be loaded at the same time. Maybe you can get it working if you use one Firefox instance in virtualisation, such as via Sandboxie, and run the other normally. Or, use an alternative Firefox like Cyberfox etc..., for special sites. But beware, some of these alternatives create their own profiles, while others use the existing Firefox profile.
As far as I know, you can do this by creating another profile where you install just the addons you want and opening another (simultaneous) instance of Firefox with this profile, with the -no-remote option. This page explains how: http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile