You do taxes online? IN Firefox?

The only option was to save as a PDF? That's strange. I have very little experience with Linux, but surely you have some program that will do PDF in Linux.

Did it specifically say it was "encrypted"? That would be even stranger. Perhaps it is just that Firefox has not been told which application to use. Browsers do not 'decrypt'.... saving the file should not be a problem, no matter what you'd use to view it.

It's important to be clear on what does what and what formats are available for saving.