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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
After official firefox send was shutdown and mozilla stop maintain it, there is one popular fork at https://gitlab.com/timvisee/send and they continue to develop it.
Looking at list of intances from https://gitlab.com/timvisee/send-instances, looks like the instance that you mention is okay.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.