Extension fails to properly spoof user-agent for snapchat

When on https://www.snapchat.com/web/, the user-agent is sometimes not spoofed. Mostly after a refresh, but it also happens that after some time the page will detect I am using firefox, without me having performed any interaction (just the tab in the background). As described in #29, a solution is Ctrl+F5 (interestingly, F5 with dev tools open and disable cache does not work), or toggling Chrome Mask off and on. Clicking on "Forget this site" then relogging into Snapchat with Chrome Mask on from the beginning does not fix the behavior, so it does not seem to be a cache problem.