Suggestion: Create a second profile. Have that profile use the VPN setting. You can use sync to share passwords, bookmarks, and history. Anything else may not be compatible between different browser versions or computer systems.

My vpn doesn’t have that feature. In browser, I can’t make the settings?

Sorry, I meant a second profile in Firefox.

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address box.

I can create profile. But I can't create those for vpn. Could you help me?

First, label/rename each new profile so it shows what VPN it is using.

Next, load each profile. Then: Type about:preferences#advanced<Enter> in the address box.

Under Advanced, Select Network.

[56+] Type about:preferences#general<enter> in the address box. Look for Network. +++++++++++++++++++++ Look for Configure How Firefox Connects and press the Settings button. If you are using a proxy, make sure those settings are correct. If there is no proxy, first try No Proxy. If there is a problem, then try System Proxy.