Windows build-in Sandbox

Firefox 57 supports only "WebExtensions". So you'll have some time before you might have to "jump ship"
But if the developers of all your installed addons provide a WebExtension-version of their addons, you can stay on Firefox.

 

How Edge's sandbox has been improved with Creators Update:
https://blogs.windows.com/msedgedev/2017/03/23/strengthening-microsoft-edge-sandbox/

 

Good article and a LOT of AppContainers. Interesting stuff, indeed. Creators Update will significantly improve the security of Edge.

Still (and always) a Chromium user here, though.

 

I would still like to be able to virtualize browsers, that's why I use SBIE. But seems it even has problems with the new FF sandbox.

 

That is the problem i had with Sbie , it uses a special mechanism to isolate application (based on some sort of whitelisting of the code), if this code changes drastically like with some of the major Chrome releases (or the newly introduced FF sandbox you have mentioned) , Sbie needs to adapt and it is not immediate, the user have to wait a new build. This is the exact reason why i ditched it despite i loved it.
I don't like to perpetually update Sbie everytime the devs have to fix an incompatibility. My system is a static one, once installed and set, i like to keep the baseline as clean and lean as possible. Reason i never had issues with Windows Updates.

 

Well, for me it's not a big problem because I don't always immediately update browsers. Older versions of Chrome, Vivaldi and Firefox are running quite smooth.

 

I take a different approach. I keep my OS native apps and don't install any third party software before I update the OS, then remove all temp files before I make a whole disk image of the OS drive. Then I keep all applications, especially browsers, up to date. Because older versions of browsers contain known security vulnerabilities, which could be too dangerous to be of production use.

 

The thing is, with Sandboxie protecting the browser, I'm not that worried about browsers being exploited. That's why I also don't really care about that Chrome and Edge are more secure than Firefox. Usability is more important to me.