Firefox extension issue.

Hello. I want to install Firefox extension from BitComet's settings, but every time I try it, I get this:



I tried with Firefox closed, I tried to run BitComet in Administrator mode; still the same results. Any ideas?

System Specs
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• OS: Windows 7 Ultimate, SP1, x64
• BitComet: 1.40, x64
• Firefox: 44.0.2