What happend to `toString` in FF74?

Is anybody aware of changes to toString introduced in the recent release of Firefox version 74?
We are using a little self-hosted AddOn that worked fine up to version 73.x, but stopped working with the new version 74. When checking the AddOn via about:debugging, I ran into “something.toString is not a function” errors.
I quickly got rid of toString (sometimes by using JSON.stringify instead, but mostly I just had to replace console.log("var = " + var) with console.log("var = ", var).
Nothing else was changed and the new version runs fine, so apparently, toString was really at fault.

However, the toString reference (last update 2020-02-13) mentions no deprecation or other significant changes; and the version 74.0 release notes do not mention anything like that either.

Am I missing something deep here?

What kind of value is var there?

I am sorry for having bothered y’all. I was just about to copy-paste the changed code directly from the diffs when I noticed that the problem was not toStringbut rather toSource and I mixed them up in the research after correcting my code.

While this is still not specifically mentioned in the release notes, at least the reference mentions not only that it is deprecated and very porly supported. A (somewhat hidden) note explicitely says

Starting in Firefox 74, toSource() is no longer available for use by web content. It is still allowed for internal and privileged code

(AddOn code is not internal or privileged in this sense).

Case closed - thanks and sorry again.

Usually you’ll want to check the “changes for developers” page on MDN or the hacks.mozilla.org blogpost for a Firefox release. The release notes are geared toward user-facing changes.

Hacks post for 74 that links to various resources like the mentioned MDN page:

I should also mention that there is a separate blog post for extension specific changes on Mozilla’s add-ons blog: