Question recommendation about setup and how to ensure full scope of potential benefits are met during implementation.
I take this to be a best practice to not use more than one vpn at a time. Should those visiting and be given access to network be asked to turn off theirs if connected to same network setup? Should any extensions intended to sync favorites or 'read later' be turns off as well?
This is something for network consultant to discuss this with you on. VPN using is based on user preferences for security and their own intranet services. Sync should only be done on their own profiles doing to others will wipe clean any profile settings their own browser saved data they saved and created. You need to talk to your IT department or Help Desk on this matter. This goes beyond Firefox software usage and help.
I assume you are running the VPN software on your computer/phone/tablet, and not on the network router. In that case, everyone sharing the same WiFi or wired network from their own device can use a different VPN, or no VPN, and there won't be a conflict.
Similarly, I don't think you need to worry about what add-ons a guest is running on their device. They won't interfere with what you are doing on your device.