Spent a lot of my time just trying to get it to work. Finally I've done it but I'm an IT technician and I believe that most of regular people will fail here.
And according to the feedbacks it seems there is a lot of such failed tryouts when people can't even start to use it. Please do something with that.
For example my case was that:
1. Different versions (64-bit FireFox against x86 KeePassXC portable). Extension runs proxy but nothing happens futher - extension doesn't see proxy running or something like that. Fail.
2. KeePassXC (installed) doesn't make org.keepassxc.keepassxc_browser_firefox.json nor apropriate registry entry - proxy doesn't run. I believe that's because of UAC and permissions. Still hothing works. Fail again.
3. Portable x64 KeePassXC + Firefox x64 - win. It creates json and write proper registry key. Extension runs proxy and they both interact each other.