Extensions and Themes are installed to the Firefox Profile. Each user logon account has its' own unique Profile. IOW, you need to install the extensions you want in each Profile.

Firefox 4 is changing the "bottom bar" - the Status bar is being replaced by an "Addons bar". You'll be able to place the "buttons" that now appear in the Status bar on any Toolbar you want them on or leave them in the Palette.

Tools > Sync = Disconnect

i want to know the place that the add-ons are being saved to, because i know for a fact it is physically saved to this computer. otherwise i would be able to see my add-ons on any computer i log into firefox with via Sync

You have to be careful with the term "Addons". That title covers Plugins, Extensions, and Themes. Plugins aren't actually "in" Firefox, they are retrieved through the Registry and aren't unique to a Profile; whereas, Extensions and Themes are unique to the Profile.

Extensions are stored in the \extensions\ folder in your Profile folder.
http://support.mozilla.com/en-us/kb/Profiles#How_to_find_your_profile

The Sync service only synchronizes specific types of data.

   * Bookmarks
   * Passwords
   * Preferences
   * History
   * Tabs 

It won't synchronize any user data specific to an installed extension. The Sync developers are working on the user being able to sync some types of "extension data", but that support has to be built into each extension - and AFAIK, none are able to do that currently.
https://support.mozilla.com/en-US/kb/What+is+Firefox+Sync