Well I guess I have my answer. Good thing I've ordered a new kindle and I'm just waiting for delivery. Thanks!According to Wikipedia, the last year the 8.9" Fire HDX 8.9 was made was 2014.
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While the Toolbox does support the HD6 and HD7 from that year, the HDX8.9 is not listed under supported models. I'm not sure what the differences are between models, but according to the OP that model isn't supported.