macos – Prevent Firefox from stealing FirefoxESR events?

tl;dr: How to run both Firefox(79 as of now), and FirefoxESR(68) at the same time, and have FirefoxESR to be the default OS browser, where all external URL cliks get open at?

For a long time, at my work computer, I have used Firefox for work and another browser for personal links (previously Opera, lately Chrome).

Recently I’ve decided to use FirefoxESR for work, and Firefox for personal links.

All seem to work fine. I still have to binaries. Two icons (with different icons since ESR uses the old one). Profiles are completely separate. etc.

The only Problem is the that Firefox(latest) keeps stealing OS events from FirefoxESR, and I want ESR, i.e. my work browser, to be the default.

I want to keep FirefoxESR as my default OS browser. On the OSX prefences window I have already set ESR instead of latest:
Selecting default browser on mac

If only one Firefox is open, it will get all the external URL clicks. If both are open (doesn’t mater which was open last, or which had focus last) external URL clicks will open a new tab on latest Firefox (the non default one!).

Already disabled the “set this browser as the default” on both browsers, and hoped the OS setting would prevail. No. Still get links on the wrong Firefox.

Is there any hope for this?