

No custom ROM on a recent smartphone technically gives you a fully open source Android system when they rely on vendor-provided proprietary blobs in order for basic hardware functionality to work at all. Unless you want to go without a modem, GPS, and likely more depending on your model, at which point it’s functionally no longer a smartphone.
Open-source custom ROMs are at least far more open-source than the alternative in most of the ways that matter most, including the ability to change the code in order to remove app installation restrictions, to avoid Google’s telemetry, etc.



The absence of JMAP support seems even more strange when considering that the Thunderbird team is planning to sell email hosting with server software (Stalwart) that not only supports JMAP but seems to have a strong focus on it.
The Thunderbird Pro team even made a blog post today with pictures for the Thundermail account dashboard, one of which shows JMAP info: https://blog.thunderbird.net/2025/11/thunderbird-pro-november-2025-update/