Bizarrely, the donation page thinks I'm in the UK, when I'm almost as far as possible away from there.
I have very strong suspicions about the engineering quality of their work (but can't confirm).
With the recent version, v78, they did broke extensions again, however, something very smell is the policy that they've adopted - the new replacement APIs are incomplete and experimental (see https://developer.thunderbird.net/add-ons/updating/tb78):
> While the Thunderbird team plans to add more APIs with upcoming releases, the current set of APIs will not be sufficient to port most add-ons. To work around this limitation, add-ons can introduce their own, additional APIs as so-called Experiments.
> As Experiments usually run in the main process and have unrestricted access to any aspect of Thunderbird, they are expected to require updates for each new version of Thunderbird. To reduce the maintenance burden in the future, it is in your own interest to use Experiment APIs only to the extent necessary for the add-on.
Essentially, they've broken compatibility by introducing a half-baked framework, and guaranteed that compatibility will be broken again and again.
I don't have the technical insight to know if this is really necessary (I know that in the past, breaking compatibility was a necessity in some cases), but on the outside, this looks like terrible, careless, engineering.
Tried browsing from google.es and again it was set to Mexico, though this time I was able to donate after changing to Spain.