You can try using ForceBindIP, there are some addons for it to have a GUI for it instead of having to use it in CMD maybe this solves your problem.
This will probably not work any more for Firefox. ForceBindIP does not catch sub-processes that get spawned and re-inject the DLL that intercepts system calls. It has not worked with Chrome for a while because Chrome has used multiple sub-processes for some time. Now Firefox does too.
I use this method: https://airvpn.org/topic/9491-guide-to-setting-up-vpn-just-for-torrenting-on-windows/?p=49046
So suppress the VPN from being the default gateway by adding routing table entries with a "192.0.0.0" mask (look at the original guide in that thread for an explanation) and run SQUID (under Cygwin) "bound" to the VPN IP interface. Then set up a Firefox profile (I believe Chrome also has profiles) to use that SQUID process as an HTTP proxy.
If there is an easier way, I have not found it.
This way works very reliably. No leaks.
If you know how to set up Linux (or BSD) on a VM (e.g. VirtualBox), then on that you could run the VPN (as the default gateway, for simplicity) and also SQUID (now with no need for a complicated configuration for SQUID). Using SQUID rather than a "dynamic" SSH connection (i.e. SOCKS) to the VM has the advantage that you can use the AirVPN DNS server - again just for that browser profile.
AirVPN has made it clear that they will not be setting up proxies. So it is a matter of doing it yourself or forgetting about it.