Alongside the Firefox browser, Mozilla also makes one of the most popular email clients for Windows out there: Thunderbird. Until now, you've needed to download the .exe for the application from Mozilla's site but it's now become available on the Microsoft Store.
The application is, however, not an official release by Mozilla. Instead, it's been repurposed and repackaged for the Microsoft Store via a developer known as Store Ports, which works to create ports of open source programs like Thunderbird for release on Microsoft's app marketplace.
The release that's currently available on the Store is based on Thunderbird 60.7 and if you're concerned about the unofficial nature of the release, you can find the source code for each release on the developer's GitHub page here.
As our Sharath Ravishankar explains, while x86 apps may not change in functionality between their regular and Microsoft Store versions, using Microsoft's Store to download apps does bring some benefits, such as a less messy installation and an easier uninstallation process. Updates to applications are also easier to perform via the Store, though in this case, you may find yourself on older Thunderbird releases for longer as the developers work on porting new releases by Mozilla.
You can grab the Microsoft Store version of Thunderbird here.