Open-source FoxyProxy add-on provides a convenient UI for managing proxies that you have access to. Profiles are supported. The company also sells paid proxy subscriptions. You can find free proxies out there too of varying (ie bad) reliability.
OpenVPN is Layer 2 or 3 so would be system-wide. If you use Whonix Workstation and are mindful of maintaining Stream Isolation, virtually the only traffic going to the VPN will be your browser traffic. (Why: Prevent Bypassing the Tunnel-Link)
You can try free 3-hr trial on Mullvad.net. (Anonymous trial, good performance, privacy-oriented - so they say). Remember to use Iceweasel. (Didn’t use to work with Tor Browser, maybe that’s changed.)
If you really need to be sure that nothing will
pass to the VPN be tunneled except browser traffic, then you could set up an SSH tunnel to serve as a socks proxy. More secure than using a socks proxy outright, but more hassle to set up.