Gloda is an interesting example case because it actually did start out its life as an extension. Apart from a dependency on Thunderbird locales for localized strings, it largely retains this structure.

While I have been plugging Jetpack for quite some time as the future of (easy) extensions for Thunderbird, there is exciting news in that Jetpack is being rebooted to be even more appropriate for the use-cases you discuss.

Namely, I think Jetpack2 (not sure if it will actually have the 2, but let’s use it for the sake of argument), will allow Thunderbird to ship with extensions built-in that are still exposed as extensions AND also provide for the user to download more recent versions of those extensions.

This would help alleviate the need to monkey-patch 3.0′s Gloda to allow other extensions to access/provide new Gloda features and functionality, like GlodaQuilla’s do-not-index feature (which is fully sanctioned monkey-patching).

Thunderbird can benefit from this! The big start was with Lightning, but I hope to see more serious development with other extensions and getting shipped with the core.