The MIME method has very strong potential and may have a low footpint to implament.
Method could include pre-recorded MIME actions in Ffox where it could: Install add-on if application/x-xpinstall;app=firefox Save to Disk if application/x-xpinstall;app=thunderbird Save to Disk if application/x-xpinstall;app=(Other Moz App name)
Alternativly if the add-in is a multi-MIME *.xpi the add-in site would offer multiple links, one link per MIME where app=xxxxx is in the link.
Method One would need minimal code support leveraging existing MIME handler. Disadvantage is local install stays like present where user need to know where d/l was saved to.
Method Two would need Add-in page Webmaster support and Evangilisem to third-party sites. For error resistance each app would need reject code that filters by parsing on the app=xxxxx portion of the MIME extension to prevent cross-app install of invalid features (bookmarks in mail, or LDAP in browser).
One of the features of MR Tech Local Install that appeals is the ability to designate archive folders for add-in's. This code should be built into Mozilla Apps with an eye to a standard location per supported platform. On Windows 9X in the folder X:\Windows\Application Data\Mozilla\Addins\(app name) where other folders at the ..\Mozilla\.. level would be Firefox, Tunderbird, etc. Then when an add-in is d/l or updates, the *.xpi would be archived and have a standard path for install. An added benifit is rollback and ease of adding add-in's to a new profile.