This article describes the major changes in Thunderbird version 10. For a complete list of bugs that have been resolved in the release, refer to the list of fixes on Bugzilla.
Add-ons are compatible by default in Thunderbird 10. Previously, add-ons were flagged as incompatible merely because the metadata for the add-on didn't explicitly include the current release number. While this was acceptable when releases were infrequent, with the new rapid release process it is difficult for add-on authors to keep the metadata updated. Because the majority of add-ons continue to function from one release to another, Thunderbird now assumes that add-ons are compatible with the updated version.
The new "Open Search" feature allows users to search the web from within Thunderbird (via numerous sites such as Google, Wikipedia, etc). To search, highlight some text in the message then right-click and select Search the web for: ... from the context menu.
A new tab will be opened that shows the search results from your default search site.
The search sites that you can search are displayed on a vertical toolbar on the left. Click the site icons to re-run the search using different search sites. Use the next and previous arrows to move backwards and forwards through your search results. Click the heart icon at the bottom of the toolbar (lower left corner of your screen) to set the selected search site as the default.
The Global Database is an index of messages that enables Global Search and other Thunderbird features, including the Conversations view add-on. In Thunderbird 10, the database size has been reduced, but the message indexes must be rebuilt. Depending on the number of messages you have and the speed of your system, you may notice a temporary slow-down during the re-index (which happens automatically in the background). Once Thunderbird has finished re-indexing, it will be as fast as it was before.