Update: The extension is now working properly with Firefox. I changed the star rating appropriately.
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I am a frequent and long-time user of PIA. It works well with native clients on all platforms and with the Chrome Extension. However, even with help from PIA Customer Support, I never got it running on Firefox. I tried many computers, many versions, fresh installed copies of everything, but it always hung during connect.

PIA works fine, just not yet on Firefox.