Recently Firefox changed how the browser works at a fundamental level in what they called Firefox Quantum or Firefox v57 and up. Despite the update one of the long term problems (to me anyway) is the issue of the open dialog box where "Do this automatically for files like this from now on." is greyed out. As a result you do not get to tell Firefox how to handles these file types (or extension and what Mozilla calls a MimeType) from now on so do not have to keep verifying if it should download automatically or open a certain program. There is a technical reason for why its greyed out but it really doesn't matter because its a server side setting that you as the user has no control over and so you can't force that option to not be greyed out. The problem is the powers that be at Mozilla have long decided that the user should not be able to manually add extensions types via the gui interface.
Fortunately there is a fix that I found by combined multiple poor answers from various forums. Why people skip all the steps when explaining how to fix something always confuses me since they are trying to be helpful but provide answers that assumes their audience mostly knows the answer and just needs a nudge in a particular direction. In the pre-quantum days, the fix involved editing the mimeTypes.rdf file. Now it involves editing a different file called handlers.json. Now these instructions are going to assume you know the basics of your computer as some parts will change based on which version of Windows you have. If not sure how to do something like open Windows Explorer or Run, google it for your operating system.
