Does literally nothing for me. I can click the extension's icon from the toolbar as described on their site... "Click the pdfFiller icon in the upper right corner. Enter your email address and password." ... but the button doesn't work, and it seems I can't use the extension at all.

Handy at times, but the permissions are far more intrusive than necessary for pdf files. Does this really need to be active for EVERY potential pdf? Does it need to obstruct the user from using Firefox browser to view local files? I need the features so rarely that if I disable the extension, I could forget it's even installed, and use another option for form filling. In sum it's more nuisance than convenience.