What is the expected behavior of the "Work Offline" mode regarding the access to a local server? To test my new Progressive Web App and its Service-Worker, I'd like to switch off temporary firefox from the network access. For this purpose, I though to use File->"Work Offline". What I observe is that in "Work Offline" mode the access to internet is disable (as I expect) but the access to my local web server for development is still avalaible (and I'm surprise about this behavior). To make sure that the Browser cache is not responsible for this behavior, I have clear the cache and disable the cache. Still I get the unexpected behavior: In "Work Offline" mode, Firefox can still request and download data from a localhost server! If I stop my localhost server, then Firefox complains about the missing data. Which is what I expect and what I also would like to see in "Work Offline" mode. What is the expected behavior of the "Work Offline" mode for the local servers? Is the current behavior a bug? I have also tried Chromium and it behaves as I expect, i.e. in "Work Offline" mode access to internet AND local servers are prevented. I'm running Ubuntu 18.04 and Firefox 67.0.3 (64-bit)